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>

api/src/rehearsalhub/repositories/band.py

@@ -81,6 +81,19 @@ class BandRepository(BaseRepository[Band]):
         result = await self.session.execute(stmt)
         return result.scalar_one_or_none()
 
+    async def get_by_nc_folder_prefix(self, path: str) -> Band | None:
+        """Return the band whose nc_folder_path is a prefix of path."""
+        stmt = select(Band).where(Band.nc_folder_path.is_not(None))
+        result = await self.session.execute(stmt)
+        bands = result.scalars().all()
+        # Longest match wins (most specific prefix)
+        best: Band | None = None
+        for band in bands:
+            folder = band.nc_folder_path  # type: ignore[union-attr]
+            if path.startswith(folder) and (best is None or len(folder) > len(best.nc_folder_path)):  # type: ignore[arg-type]
+                best = band
+        return best
+
     async def list_for_member(self, member_id: uuid.UUID) -> list[Band]:
         stmt = (
             select(Band)