feat: search Party songs and enqueue on result selection

[amitbreuer]

Stack

Depends on #5. The base is amit-breuer-zoominfo-feat-free-apple-song-lookup at 8eb573c, intentionally not main. This PR contains the song-search/selection change only.

Summary

• Separate debounced song-link resolution from queue mutation: searching creates no requests or jobs.
• Show compact shared song cards; selecting one enqueues without an extra approval step or confirmation dialog.
• Validate signed, five-minute, room/session/source-bound result capabilities and atomically deduplicate selection using a shared search nonce. Preserve recording and pre-send safety checks.
• Guard stale search results and clicks. Show Added to the queue only for the current guest request after confirmed Spotify enqueue success; distinguish Not found from actual errors.
• Add guest preview scenarios and development-only jsdom coverage.

Validation reported by implementation session

• Five workspace builds, 103 tests (20 API, 40 provider, 18 PostgreSQL, 25 frontend including six React DOM tests), and focused lint passed.
• HTTP preview responds: http://127.0.0.1:5213/app/?section=party&demo=guest
• Actual browser rendering and keyboard verification remain blocked by Chrome EPERM, Chromium SIGSEGV and launch timeout failures. DOM tests and HTTP availability are not a substitute for browser verification.

Release boundaries

No live provider calls, real enqueue, cloud changes, deployment or production migrations. Existing release gates remain, including real Telegram/provider verification and infrastructure setup. Aggregate iTunes quota/cache enforcement remains a documented limitation.