feat: incremental SSE scan, recursive NC traversal, custom folder support

- nc_scan.py: recursive collect_audio_files (fixes depth-1 bug); scan_band_folder
  yields ndjson events (progress/song/session/skipped/done) for streaming
- songs.py: replace old flat scan with scan_band_folder; add GET nc-scan/stream
  endpoint using _member_from_request so ?token= auth works for fetch-based SSE
- BandPage.tsx: scan button now consumes ndjson stream via fetch+ReadableStream;
  sessions/unattributed invalidated as each song/session event arrives
- session.py: add extract_session_folder() for YYMMDD path extraction
- rehearsal_session.py: get_or_create uses begin_nested() savepoint to handle races
- band.py: add get_by_nc_folder_prefix() for custom nc_folder_path band lookup
- internal.py: nc-upload falls back to prefix match when slug lookup fails
- event_loop.py: remove hardcoded bands/ guard; let internal API handle filtering

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

watcher/src/watcher/event_loop.py

@@ -66,11 +66,6 @@ def normalize_nc_path(raw_path: str, username: str) -> str:
     return path
 
 
-def is_band_audio_path(path: str) -> bool:
-    """True if the user-relative path is inside a band folder."""
-    parts = path.strip("/").split("/")
-    return len(parts) >= 2 and parts[0] == "bands"
-
 
 def extract_nc_file_path(activity: dict[str, Any]) -> str | None:
     """Extract the server-relative file path from an activity event."""
@@ -140,10 +135,6 @@ async def poll_once(nc_client: NextcloudWatcherClient, settings: WatcherSettings
             )
             continue
 
-        if not is_band_audio_path(nc_path):
-            log.info("  → skip: path not inside a bands/ folder")
-            continue
-
         if activity_type not in _UPLOAD_TYPES and subject not in _UPLOAD_SUBJECTS:
             log.info(
                 "  → skip: type=%r subject=%r is not a file upload event",