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/db/models.py

@@ -207,6 +207,7 @@ class SongComment(Base):
     )
     body: Mapped[str] = mapped_column(Text, nullable=False)
     timestamp: Mapped[Optional[float]] = mapped_column(Numeric(10, 2), nullable=True)
+    tag: Mapped[Optional[str]] = mapped_column(String(32), nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), nullable=False
     )