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/schemas/comment.py

@@ -9,6 +9,7 @@ from pydantic import BaseModel, ConfigDict
 class SongCommentCreate(BaseModel):
     body: str
     timestamp: float | None = None
+    tag: str | None = None
 
 
 class SongCommentRead(BaseModel):
@@ -21,6 +22,7 @@ class SongCommentRead(BaseModel):
     author_name: str
     author_avatar_url: str | None
     timestamp: float | None
+    tag: str | None
     created_at: datetime
 
     @classmethod
@@ -33,5 +35,6 @@ class SongCommentRead(BaseModel):
             author_name=getattr(getattr(c, "author"), "display_name"),
             author_avatar_url=getattr(getattr(c, "author"), "avatar_url"),
             timestamp=getattr(c, "timestamp"),
+            tag=getattr(c, "tag", None),
             created_at=getattr(c, "created_at"),
         )