feat: infrastructure hardening — WS subscriber refactor, DB pooling, HNSW indexes

[CodeNinjaSarthak]

• Refactor Redis pub/sub relay into WebSocketManager.start_subscriber() with exponential backoff (1→30s) and auto-reconnect on failure
  • Fix Redis channel prefix: ws:session:{id} → ws:{id} (manager + posting worker)
  • Add publish() helper on WebSocketManager for future use
  • Tune DB connection pool per worker: pool_size=2, max_overflow=3 (budget: 45 total)
  • Add Alembic migration c3d4e5f6a7b8: HNSW indexes on comments and rag_documents embeddings
  • Add scripts/truncate_embeddings.sql utility to clear embeddings before re-indexing
  • Set postgres max_connections=100 in docker-compose
  • Switch Redis maxmemory-policy from allkeys-lru to volatile-lru (preserve non-expiring keys)
  • Run uvicorn with --workers 2 in api.Dockerfile
  • Remove legacy vanilla-JS frontend files (replaced by React/Vite)