Migrate backend from Python to Java Spring Boot

- Full Java 21 + Spring Boot 3.3 backend with Virtual Threads
- HikariCP connection pool for Oracle ADB
- JWT auth, Redis caching, OCI GenAI integration
- YouTube transcript extraction via API + Playwright browser fallback
- SSE streaming for bulk operations
- Scheduled daemon for channel scanning/video processing
- Mobile UI: collapse restaurant list to single row on selection
- Switch PM2 ecosystem config to Java backend

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

.gitignore

@@ -6,3 +6,7 @@ node_modules/
 .next/
 .env.local
 *.log
+
+# Java backend
+backend-java/build/
+backend-java/.gradle/