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/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