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/alembic/versions/0005_comment_tag.py

@@ -0,0 +1,25 @@
+"""Add tag column to song_comments
+
+Revision ID: 0005_comment_tag
+Revises: 0004_rehearsal_sessions
+Create Date: 2026-04-06
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0005_comment_tag"
+down_revision = "0004"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "song_comments",
+        sa.Column("tag", sa.String(length=32), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("song_comments", "tag")