feat(remotion): audio layer — self-authored music bed + transition SFX in FlexStory

Adds audio to the scene engine without any third-party/geo-blocked sourcing: the
beds + SFX are synthesized with ffmpeg, so they're license-free (CC0, self-authored)
and need no acquisition — the same play as self-authoring Lottie.

- public/audio/: music-ambient.mp3 (soft 3-tone pad, looped) + sfx-whoosh/pop/chime.
- FlexStory: optional music/musicVolume/sfx props (optional so the existing render
  binding needs no change). Renders <Audio loop> for the bed + a whoosh at each
  scene start and a chime on the final scene, driven by precomputed scene starts.
- check-assets: now also scans public/audio (+ lottie) with folder-prefixed keys;
  assets.json ledgers the 4 audio files (CC0 self-authored).

Verified: tsc clean; a 6s FlexStory render produces an MP4 with a real audio stream
(ffprobe: codec_type=audio). NOTE: these are placeholder/SFX-grade; a premium
curated music library (by vibe) is a separate sourcing sweep, and the studio music
picker → FlexStory `music` prop is a follow-up wiring.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/remotion/public/illustrations/assets.json

@@ -358,5 +358,45 @@
     "generator": "createAvatar(openPeeps,{seed:'flatrender-peep-30'})",
     "url": "https://www.dicebear.com/styles/open-peeps/",
     "vendored": "2026-06-22"
+  },
+  "audio/music-ambient.mp3": {
+    "source": "FlatRender (self-authored, ffmpeg synthesis)",
+    "license": "CC0-1.0",
+    "license_class": "CC0",
+    "commercial_ok": true,
+    "attribution_required": false,
+    "ai_training_allowed": true,
+    "note": "ambient pad music bed (looped)",
+    "vendored": "2026-06-23"
+  },
+  "audio/sfx-whoosh.mp3": {
+    "source": "FlatRender (self-authored, ffmpeg synthesis)",
+    "license": "CC0-1.0",
+    "license_class": "CC0",
+    "commercial_ok": true,
+    "attribution_required": false,
+    "ai_training_allowed": true,
+    "note": "scene-transition whoosh",
+    "vendored": "2026-06-23"
+  },
+  "audio/sfx-pop.mp3": {
+    "source": "FlatRender (self-authored, ffmpeg synthesis)",
+    "license": "CC0-1.0",
+    "license_class": "CC0",
+    "commercial_ok": true,
+    "attribution_required": false,
+    "ai_training_allowed": true,
+    "note": "accent pop",
+    "vendored": "2026-06-23"
+  },
+  "audio/sfx-chime.mp3": {
+    "source": "FlatRender (self-authored, ffmpeg synthesis)",
+    "license": "CC0-1.0",
+    "license_class": "CC0",
+    "commercial_ok": true,
+    "attribution_required": false,
+    "ai_training_allowed": true,
+    "note": "outro chime",
+    "vendored": "2026-06-23"
   }
 }