Upload application file

Upload a mobile application file (APK/IPA/AAB) to QualGent.

**How it works:**
- Server detects file size as it streams in
- Files < 32MB: Processed directly (fast path)
- Files ≥ 32MB: Automatically chunked server-side (no client chunking needed)
- **AAB files**: Converted to universal APK before storage when conversion is enabled; only the APK is stored. If conversion fails, the AAB is stored as-is. Other file types are stored unchanged.

**Benefits:**
- Simple: Just upload like any normal file
- Works with standard multipart/form-data
- No client-side chunking logic required
- Compatible with curl, Postman, fetch, etc.

**Form Data:**
- `file`: The file to upload (multipart/form-data)
- `app_name`: Application name (required)
- `version`: Application version (required)
- `os`: (optional) Operating system name (auto-inferred from file extension if not provided)

**Storage Path Format:**
- `{user_id}/{timestamp}-{app_name}-{version}.{ext}` (ext is .apk when AAB was converted)