Fix: Pass FASTAPI_URL as build arg for Next.js rewrites

Next.js rewrites are evaluated at build time, not runtime.
Without FASTAPI_URL set during build, the rewrite destination
defaults to localhost:8000 which fails in Docker.

- Add FASTAPI_URL build arg to nextjs-app/Dockerfile
- Pass build arg in docker-compose.yml
- Pass build arg in Gitea Actions workflow

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nextjs-app/Dockerfile

@@ -22,6 +22,10 @@ COPY . .
 ENV NEXT_TELEMETRY_DISABLED=1
 ENV NODE_ENV=production
 
+# Build argument for FastAPI URL (used by Next.js rewrites at build time)
+ARG FASTAPI_URL=http://backend:80/api
+ENV FASTAPI_URL=${FASTAPI_URL}
+
 # Build application
 RUN npm run build