Add Open Graph and Twitter Card meta-tags for link previews

Replace PathResourceResolver SPA fallback with SpaController that
injects OG/Twitter meta-tags into cached index.html template.
Event pages get event-specific tags (title, date, location),
all other pages get generic fete branding. Includes og-image.png
brand asset and forward-headers-strategy for proxy support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/main/java/de/fete/adapter/in/web/SpaController.java

@@ -0,0 +1,188 @@
+package de.fete.adapter.in.web;
+
+import de.fete.domain.model.Event;
+import de.fete.domain.model.EventToken;
+import de.fete.domain.port.in.GetEventUseCase;
+import jakarta.annotation.PostConstruct;
+import jakarta.servlet.http.HttpServletRequest;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.time.ZonedDateTime;
+import java.time.format.DateTimeFormatter;
+import java.util.LinkedHashMap;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Optional;
+import java.util.UUID;
+import org.springframework.core.io.ClassPathResource;
+import org.springframework.http.MediaType;
+import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.ResponseBody;
+import org.springframework.web.servlet.support.ServletUriComponentsBuilder;
+
+/** Serves the SPA index.html with injected Open Graph and Twitter Card meta-tags. */
+@Controller
+public class SpaController {
+
+  private static final String PLACEHOLDER = "<!-- OG_META_TAGS -->";
+  private static final int MAX_TITLE_LENGTH = 70;
+  private static final int MAX_DESCRIPTION_LENGTH = 200;
+  private static final String GENERIC_TITLE = "fete";
+  private static final String GENERIC_DESCRIPTION =
+      "Privacy-focused event planning. Create and share events without accounts.";
+  private static final DateTimeFormatter DATE_FORMAT =
+      DateTimeFormatter.ofPattern("EEEE, MMMM d, yyyy 'at' h:mm a", Locale.ENGLISH);
+
+  private final GetEventUseCase getEventUseCase;
+  private String htmlTemplate;
+
+  /** Creates a new SpaController. */
+  public SpaController(GetEventUseCase getEventUseCase) {
+    this.getEventUseCase = getEventUseCase;
+  }
+
+  /** Loads and caches the index.html template at startup. */
+  @PostConstruct
+  void loadTemplate() throws IOException {
+    var resource = new ClassPathResource("/static/index.html");
+    if (resource.exists()) {
+      htmlTemplate = resource.getContentAsString(StandardCharsets.UTF_8);
+    }
+  }
+
+  /** Serves SPA HTML with generic meta-tags for non-event routes. */
+  @GetMapping(
+      value = {"/", "/create", "/events"},
+      produces = MediaType.TEXT_HTML_VALUE
+  )
+  @ResponseBody
+  public String serveGenericPage(HttpServletRequest request) {
+    if (htmlTemplate == null) {
+      return "";
+    }
+    String baseUrl = getBaseUrl(request);
+    return htmlTemplate.replace(PLACEHOLDER, renderTags(buildGenericMeta(baseUrl)));
+  }
+
+  /** Serves SPA HTML with event-specific meta-tags. */
+  @GetMapping(
+      value = "/events/{token}",
+      produces = MediaType.TEXT_HTML_VALUE
+  )
+  @ResponseBody
+  public String serveEventPage(@PathVariable String token,
+      HttpServletRequest request) {
+    if (htmlTemplate == null) {
+      return "";
+    }
+    String baseUrl = getBaseUrl(request);
+    Map<String, String> meta = resolveEventMeta(token, baseUrl);
+    return htmlTemplate.replace(PLACEHOLDER, renderTags(meta));
+  }
+
+  // --- Meta-tag composition ---
+
+  private Map<String, String> buildEventMeta(Event event, String baseUrl) {
+    var tags = new LinkedHashMap<String, String>();
+    String title = truncateTitle(event.getTitle());
+    String description = formatDescription(event);
+    tags.put("og:title", title);
+    tags.put("og:description", description);
+    tags.put("og:url", baseUrl + "/events/" + event.getEventToken().value());
+    tags.put("og:type", "website");
+    tags.put("og:site_name", GENERIC_TITLE);
+    tags.put("og:image", baseUrl + "/og-image.png");
+    tags.put("twitter:card", "summary");
+    tags.put("twitter:title", title);
+    tags.put("twitter:description", description);
+    return tags;
+  }
+
+  private Map<String, String> buildGenericMeta(String baseUrl) {
+    var tags = new LinkedHashMap<String, String>();
+    tags.put("og:title", GENERIC_TITLE);
+    tags.put("og:description", GENERIC_DESCRIPTION);
+    tags.put("og:url", baseUrl);
+    tags.put("og:type", "website");
+    tags.put("og:site_name", GENERIC_TITLE);
+    tags.put("og:image", baseUrl + "/og-image.png");
+    tags.put("twitter:card", "summary");
+    tags.put("twitter:title", GENERIC_TITLE);
+    tags.put("twitter:description", GENERIC_DESCRIPTION);
+    return tags;
+  }
+
+  private Map<String, String> resolveEventMeta(String token, String baseUrl) {
+    try {
+      UUID uuid = UUID.fromString(token);
+      Optional<Event> event =
+          getEventUseCase.getByEventToken(new EventToken(uuid));
+      if (event.isPresent()) {
+        return buildEventMeta(event.get(), baseUrl);
+      }
+    } catch (IllegalArgumentException ignored) {
+      // Invalid UUID — fall back to generic
+    }
+    return buildGenericMeta(baseUrl);
+  }
+
+  // --- Description formatting ---
+
+  private String truncateTitle(String title) {
+    if (title.length() <= MAX_TITLE_LENGTH) {
+      return title;
+    }
+    return title.substring(0, MAX_TITLE_LENGTH - 3) + "...";
+  }
+
+  private String formatDescription(Event event) {
+    ZonedDateTime zoned = event.getDateTime().atZoneSameInstant(event.getTimezone());
+    var sb = new StringBuilder();
+    sb.append("📅 ").append(zoned.format(DATE_FORMAT));
+
+    if (event.getLocation() != null && !event.getLocation().isBlank()) {
+      sb.append(" · 📍 ").append(event.getLocation());
+    }
+
+    if (event.getDescription() != null && !event.getDescription().isBlank()) {
+      sb.append(" — ").append(event.getDescription());
+    }
+
+    String result = sb.toString();
+    if (result.length() > MAX_DESCRIPTION_LENGTH) {
+      return result.substring(0, MAX_DESCRIPTION_LENGTH - 3) + "...";
+    }
+    return result;
+  }
+
+  // --- HTML rendering ---
+
+  private String renderTags(Map<String, String> tags) {
+    var sb = new StringBuilder();
+    for (var entry : tags.entrySet()) {
+      String key = entry.getKey();
+      String value = escapeHtml(entry.getValue());
+      String attr = key.startsWith("twitter:") ? "name" : "property";
+      sb.append("<meta ").append(attr).append("=\"").append(key)
+          .append("\" content=\"").append(value).append("\">\n");
+    }
+    return sb.toString().stripTrailing();
+  }
+
+  private String escapeHtml(String input) {
+    return input
+        .replace("&", "&amp;")
+        .replace("\"", "&quot;")
+        .replace("<", "&lt;")
+        .replace(">", "&gt;");
+  }
+
+  private String getBaseUrl(HttpServletRequest request) {
+    return ServletUriComponentsBuilder.fromRequestUri(request)
+        .replacePath("")
+        .build()
+        .toUriString();
+  }
+}

backend/src/main/java/de/fete/config/WebConfig.java

@@ -1,21 +1,17 @@
 package de.fete.config;
 
-import java.io.IOException;
 import java.time.Clock;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
-import org.springframework.core.io.ClassPathResource;
-import org.springframework.core.io.Resource;
 import org.springframework.web.bind.annotation.RestController;
 import org.springframework.web.servlet.config.annotation.PathMatchConfigurer;
-import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
-import org.springframework.web.servlet.resource.PathResourceResolver;
 
-/** Configures API path prefix and SPA static resource serving. */
+/** Configures API path prefix. Static resources served by default Spring Boot handler. */
 @Configuration
 public class WebConfig implements WebMvcConfigurer {
 
+  /** Provides a system clock bean for time-dependent services. */
   @Bean
   Clock clock() {
     return Clock.systemDefaultZone();
@@ -25,23 +21,4 @@ public class WebConfig implements WebMvcConfigurer {
   public void configurePathMatch(PathMatchConfigurer configurer) {
     configurer.addPathPrefix("/api", c -> c.isAnnotationPresent(RestController.class));
   }
-
-  @Override
-  public void addResourceHandlers(ResourceHandlerRegistry registry) {
-    registry.addResourceHandler("/**")
-        .addResourceLocations("classpath:/static/")
-        .resourceChain(true)
-        .addResolver(new PathResourceResolver() {
-          @Override
-          protected Resource getResource(String resourcePath,
-              Resource location) throws IOException {
-            Resource requested = location.createRelative(resourcePath);
-            if (requested.exists() && requested.isReadable()) {
-              return requested;
-            }
-            Resource index = new ClassPathResource("/static/index.html");
-            return (index.exists() && index.isReadable()) ? index : null;
-          }
-        });
-  }
 }

backend/src/main/resources/application.properties

@@ -7,6 +7,9 @@ spring.jpa.open-in-view=false
 # Liquibase
 spring.liquibase.change-log=classpath:db/changelog/db.changelog-master.xml
 
+# Proxy headers
+server.forward-headers-strategy=framework
+
 # Actuator
 management.endpoints.web.exposure.include=health
 management.endpoint.health.show-details=never