T-5: set up API-first tooling with OpenAPI spec as single source of truth

Backend: openapi-generator-maven-plugin generates Spring interfaces and DTOs
from the spec. Frontend: openapi-typescript + openapi-fetch provide type-safe
API access. Both sides get compile-time contract enforcement from a single
api.yaml file.

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

backend/pom.xml

@@ -32,6 +32,11 @@
             <artifactId>spring-boot-starter-actuator</artifactId>
         </dependency>
 
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-validation</artifactId>
+        </dependency>
+
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-test</artifactId>
@@ -96,6 +101,7 @@
                     <threshold>Low</threshold>
                     <xmlOutput>true</xmlOutput>
                     <failOnError>true</failOnError>
+                    <excludeFilterFile>spotbugs-exclude.xml</excludeFilterFile>
                 </configuration>
                 <executions>
                     <execution>
@@ -105,6 +111,56 @@
                     </execution>
                 </executions>
             </plugin>
+            <plugin>
+                <groupId>org.openapitools</groupId>
+                <artifactId>openapi-generator-maven-plugin</artifactId>
+                <version>7.20.0</version>
+                <executions>
+                    <execution>
+                        <goals>
+                            <goal>generate</goal>
+                        </goals>
+                        <configuration>
+                            <inputSpec>${project.basedir}/src/main/resources/openapi/api.yaml</inputSpec>
+                            <generatorName>spring</generatorName>
+                            <apiPackage>de.fete.adapter.in.web.api</apiPackage>
+                            <modelPackage>de.fete.adapter.in.web.model</modelPackage>
+                            <generateSupportingFiles>true</generateSupportingFiles>
+                            <supportingFilesToGenerate>ApiUtil.java</supportingFilesToGenerate>
+                            <configOptions>
+                                <interfaceOnly>true</interfaceOnly>
+                                <useSpringBoot3>true</useSpringBoot3>
+                                <useBeanValidation>true</useBeanValidation>
+                                <performBeanValidation>true</performBeanValidation>
+                                <openApiNullable>false</openApiNullable>
+                                <skipDefaultInterface>true</skipDefaultInterface>
+                                <useResponseEntity>true</useResponseEntity>
+                                <documentationProvider>none</documentationProvider>
+                                <annotationLibrary>none</annotationLibrary>
+                            </configOptions>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+            <plugin>
+                <groupId>org.codehaus.mojo</groupId>
+                <artifactId>build-helper-maven-plugin</artifactId>
+                <version>3.6.0</version>
+                <executions>
+                    <execution>
+                        <id>add-openapi-sources</id>
+                        <phase>generate-sources</phase>
+                        <goals>
+                            <goal>add-source</goal>
+                        </goals>
+                        <configuration>
+                            <sources>
+                                <source>${project.build.directory}/generated-sources/openapi/src/main/java</source>
+                            </sources>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
             <plugin>
                 <groupId>org.springframework.boot</groupId>
                 <artifactId>spring-boot-maven-plugin</artifactId>

backend/spotbugs-exclude.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<FindBugsFilter>
+    <!-- Exclude generated OpenAPI code from SpotBugs analysis -->
+    <Match>
+        <Package name="de.fete.adapter.in.web.api"/>
+    </Match>
+    <Match>
+        <Package name="de.fete.adapter.in.web.model"/>
+    </Match>
+</FindBugsFilter>

backend/src/main/resources/application.properties

@@ -1,4 +1,5 @@
 spring.application.name=fete
+server.servlet.context-path=/api
 
 management.endpoints.web.exposure.include=health
 management.endpoint.health.show-details=never

backend/src/main/resources/openapi/api.yaml

@@ -0,0 +1,37 @@
+openapi: 3.1.0
+info:
+  title: fete API
+  description: Privacy-focused event announcements and RSVPs
+  version: 0.1.0
+  license:
+    name: GPL-3.0-or-later
+    identifier: GPL-3.0-or-later
+
+servers:
+  - url: /api
+
+paths:
+  /health:
+    get:
+      operationId: getHealth
+      summary: Health check
+      tags:
+        - health
+      responses:
+        "200":
+          description: Service is healthy
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/HealthResponse"
+
+components:
+  schemas:
+    HealthResponse:
+      type: object
+      required:
+        - status
+      properties:
+        status:
+          type: string
+          example: UP