feat: add OpenAPI spec (dev only) with typed frontend client

- Add springdoc-openapi 3.0.2 (supports Spring Boot 4) to backend
- Disable api-docs/swagger-ui in application.yaml (prod default)
- Enable both in application-dev.yaml; Swagger UI at /swagger-ui.html
- Add openapi-fetch (runtime) and openapi-typescript (dev) to frontend
- Add generate:api npm script — run with backend up to regenerate types
- Add src/lib/api.server.ts typed client factory (uses SvelteKit fetch)
- Gitignore src/lib/generated/api.ts (generated artifact)

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

backend/src/main/resources/application-dev.yaml

@@ -1,3 +1,10 @@
 spring:
   jpa:
     show-sql: true
+
+springdoc:
+  api-docs:
+    enabled: true
+  swagger-ui:
+    enabled: true
+    path: /swagger-ui.html