fix: use float literals for progress_pct to satisfy beartype (#634)

beartype runtime type checking requires exact type matches.
`progress_pct=0` (int) fails validation against `float | None`.

fixes upload failure: "BeartypeCallHintParamViolation: parameter
progress_pct=0 violates type hint float | None"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <[e-mail]>

backend/src/backend/_internal/background_tasks.py

···
                 export_id,
                 JobStatus.PROCESSING,
                 f"downloading {total} tracks...",
-                progress_pct=0,
+                progress_pct=0.0,
                 result={"processed_count": 0, "total_count": total},
             )
 
···
                 export_id,
                 JobStatus.PROCESSING,
                 "creating zip archive...",
-                progress_pct=100,
+                progress_pct=100.0,
                 result={
                     "processed_count": len(successful_downloads),
                     "total_count": total,

backend/src/backend/api/tracks/uploads.py

···
         JobStatus.PROCESSING,
         "uploading to storage...",
         phase="upload",
-        progress_pct=0,
+        progress_pct=0.0,
     )
     try:
         async with R2ProgressTracker(