feat(sing): order "Your songs tonight" by queue position + clear sung songs (v0.88.0)

[beveradb]

Summary

Two fixes to the singer-facing "Your songs tonight" done screen (sing.nomadkaraoke.com):

• Ordering: the list rendered in submission order, so a reordered queue looked wrong. It now sorts into the order songs will actually be sung — 🎤 now singing → queue position → songs still awaiting the KJ.
• Sung songs never cleared: a sing_request stays approved after its rotation entry is sung, so sung songs lingered (as a misleading "Added to the queue") and the list grew all night. Sung songs now move into a collapsed "✓ Already sung tonight (N)" section.

Changes

• Backend (sing.py /my-requests): add a per-song performed flag, derived from the linked rotation entry being Done/Left (targeted get_entry lookup for ids not in the active queue); no stale estimate attached for performed songs.
• Frontend (sing.js): _splitAndSortSongs + _activeSortKey sort the active list; _renderSungSection renders the collapsed read-only history (open/closed state preserved across the 15s poll); performed songs are read-only (no cancel/change/reorder); _liveSongs excludes performed so the "🎤 My songs (N)" bar count and boot smart-restore ignore sung songs (a singer who's finished lands on the request screen, not a stale list).
• CSS (sing.css): style the collapsed sung section.
• Docs (docs/ARCHITECTURE.md): document the new response shape + behavior.
• Version bumped 0.87.0 → 0.88.0 (cache-bust; backend change → device needs a restart to take effect).

Testing

• 1 new integration test (performed flag on a Done entry) + 4 new Playwright e2e tests (queue-order sort, sung-song collapse, read-only sung cards, no-section-when-none)
• All 28 sing e2e + full sing suite green
• The 5 repo-wide failures are pre-existing/environment (KJ-admin e2e drift + macOS-no-systemctl audio test); verified they also fail on the stashed pre-change tree

Review

• CodeRabbit CLI attempted (hit OSS rate limit) → local /code-review fallback: no correctness bugs; only low-severity cosmetic/efficiency notes, none warranting a change

@coderabbitai ignore

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