Rework song player view to match design system

- New split layout: waveform/transport/queue left, comment panel right
- Avatar pins above waveform positioned by timestamp with hover tooltips
- Transport bar: speed selector, ±30s skip, 46px amber play/pause, volume
- Comment compose: live timestamp pill, suggestion/issue/keeper tag buttons
- Comment list: per-author colour avatars, amber timestamp seek chips,
  playhead-proximity highlight, delete only shown on own comments
- Queue panel showing other songs in the same session
- Waveform colours updated to amber/dim palette (104px height)
- Add GET /songs/{song_id} endpoint for song metadata
- Add tag field to SongComment (model, schema, router, migration 0005)
- Fix migration 0005 down_revision to use short ID "0004"
- Fix ESLint no-unused-expressions in keyboard shortcut handler

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

api/src/rehearsalhub/routers/songs.py

@@ -89,6 +89,24 @@ async def search_songs(
     ]
 
 
+@router.get("/songs/{song_id}", response_model=SongRead)
+async def get_song(
+    song_id: uuid.UUID,
+    session: AsyncSession = Depends(get_session),
+    current_member: Member = Depends(get_current_member),
+):
+    song_repo = SongRepository(session)
+    song = await song_repo.get_with_versions(song_id)
+    if song is None:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="Song not found")
+    band_svc = BandService(session)
+    try:
+        await band_svc.assert_membership(song.band_id, current_member.id)
+    except PermissionError:
+        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Not a member")
+    return SongRead.model_validate(song).model_copy(update={"version_count": len(song.versions)})
+
+
 @router.patch("/songs/{song_id}", response_model=SongRead)
 async def update_song(
     song_id: uuid.UUID,
@@ -264,7 +282,7 @@ async def create_comment(
 ):
     await _assert_song_membership(song_id, current_member.id, session)
     repo = CommentRepository(session)
-    comment = await repo.create(song_id=song_id, author_id=current_member.id, body=data.body, timestamp=data.timestamp)
+    comment = await repo.create(song_id=song_id, author_id=current_member.id, body=data.body, timestamp=data.timestamp, tag=data.tag)
     comment = await repo.get_with_author(comment.id)
     return SongCommentRead.from_model(comment)