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>

packages/shared/src/validation/track.ts

@@ -53,6 +53,20 @@ export const updateVersionSchema = z.object({
   branchLabel: z.string().max(100).nullable().optional(),
 });
 
+export const requestStemUploadUrlSchema = z.object({
+  fileName: z.string().min(1),
+  mimeType: z.string().min(1),
+  fileSize: z.number().int().positive().max(MAX_FILE_SIZE),
+});
+
+export const createStemSchema = z.object({
+  fileKey: z.string().min(1),
+  name: z.string().min(1).max(255),
+  originalFileName: z.string().min(1),
+  mimeType: z.string().min(1),
+  fileSize: z.number().int().positive(),
+});
+
 export const createShareLinkSchema = z.object({
   expiresAt: z.string().datetime().optional(),
   allowComments: z.boolean().optional(),
@@ -75,3 +89,5 @@ export type UpdateVersionInput = z.infer<typeof updateVersionSchema>;
 export type CreateShareLinkInput = z.infer<typeof createShareLinkSchema>;
 export type CoverUploadInput = z.infer<typeof coverUploadSchema>;
 export type GuestCommentInput = z.infer<typeof guestCommentSchema>;
+export type RequestStemUploadUrlInput = z.infer<typeof requestStemUploadUrlSchema>;
+export type CreateStemInput = z.infer<typeof createStemSchema>;