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>

backend-java/start.sh

@@ -0,0 +1,10 @@
+#!/bin/bash
+export JAVA_HOME="/opt/homebrew/opt/openjdk@21/libexec/openjdk.jdk/Contents/Home"
+export PATH="/opt/homebrew/opt/openjdk@21/bin:$PATH"
+
+# Load env from Python backend (shared env)
+set -a
+source /Users/joungmin/workspaces/tasteby/backend/.env
+set +a
+
+exec ./gradlew bootRun