feat: add STEM file support per track

- DB: stems table with trackId FK, fileKey, sortOrder, createdById
- API: GET/POST/DELETE stems, presigned upload URL, ZIP download via fflate
- Web: StemUploadDropzone (multi-file, batch upload, progress bars)
- Web: StemList with download-all-ZIP and per-stem delete
- Web: STEMs tab in track detail view
- Icon: add 'music' icon to inline set
- Auto-migration runs stems table creation on boot

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/package.json

@@ -14,6 +14,7 @@
     "@music-hub/db": "workspace:*",
     "@music-hub/shared": "workspace:*",
     "drizzle-orm": "^0.44",
+    "fflate": "^0.8.2",
     "hono": "^4",
     "resend": "^6.10.0"
   }