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>

worker/src/worker/pipeline/waveform.py

@@ -7,7 +7,7 @@ import json
 import numpy as np
 
 
-def extract_peaks(audio: np.ndarray, num_points: int = 1000) -> list[float]:
+def extract_peaks(audio: np.ndarray, num_points: int = 500) -> list[float]:
     """
     Downsample audio to `num_points` RMS+peak values for waveform display.
     Returns a flat list of [peak, peak, ...] normalized to 0-1.