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/rehearsal_session.py

@@ -40,11 +40,19 @@ class RehearsalSessionRepository(BaseRepository[RehearsalSession]):
         existing = await self.get_by_band_and_date(band_id, session_date)
         if existing is not None:
             return existing
-        return await self.create(
-            band_id=band_id,
-            date=datetime(session_date.year, session_date.month, session_date.day),
-            nc_folder_path=nc_folder_path,
-        )
+        try:
+            async with self.session.begin_nested():
+                return await self.create(
+                    band_id=band_id,
+                    date=datetime(session_date.year, session_date.month, session_date.day),
+                    nc_folder_path=nc_folder_path,
+                )
+        except Exception:
+            # Another request raced us — fetch the row that now exists
+            existing = await self.get_by_band_and_date(band_id, session_date)
+            if existing is not None:
+                return existing
+            raise
 
     async def list_for_band(self, band_id: uuid.UUID) -> list[tuple[RehearsalSession, int]]:
         """Return (session, recording_count) tuples, newest date first."""