fix: robust NC activity filter, title extraction, scan result detail

Watcher:
- Accept both NC 22+ (type="file_created") and older NC (subject="created_self")
  so the upload filter works across all Nextcloud versions
- Add .opus to audio_extensions
- Fix tests: set nc.username on mocks, use realistic activity dicts with type field
- Add tests for old NC style, non-band path filter, normalize_nc_path, cursor advance

API:
- Fix internal.py title extraction: always use filename stem (was using
  parts[-2] for >3-part paths, which gave folder name instead of song title)
- nc-scan now returns NcScanResult with folder, files_found, imported, skipped counts
  instead of bare song list — gives the UI actionable feedback

Web:
- Show rich scan result message: folder scanned, count imported, count already registered

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

api/src/rehearsalhub/routers/internal.py

@@ -53,12 +53,17 @@ async def nc_upload(
         log.warning("nc-upload: band slug '%s' not found in DB", band_slug)
         return {"status": "skipped", "reason": "band not found"}
 
-    # Determine song title and folder from remaining path segments
-    # e.g. bands/my-band/songs/session1/rec.mp3 → folder=bands/my-band/songs/session1/, title=session1
-    # e.g. bands/my-band/rec.mp3 → folder=bands/my-band/, title=rec
+    # Determine song title and folder from path.
+    # The title is always the filename stem (e.g. "take1" from "take1.wav").
+    # The nc_folder groups all versions of the same recording (the parent directory).
+    #
+    # Examples:
+    #   bands/my-band/take1.wav            → folder=bands/my-band/,         title=take1
+    #   bands/my-band/231015/take1.wav     → folder=bands/my-band/231015/,   title=take1
+    #   bands/my-band/songs/groove/take1.wav → folder=bands/my-band/songs/groove/, title=take1
     parent = str(Path(path).parent)
     nc_folder = parent.rstrip("/") + "/"
-    title = Path(path).stem if len(parts) == 3 else parts[-2]
+    title = Path(path).stem
 
     version_repo = AudioVersionRepository(session)
     if event.nc_file_etag and await version_repo.get_by_etag(event.nc_file_etag):