feat(api): auto-link rehearsal sessions on watcher upload and nc-scan

parse_rehearsal_date() extracts YYMMDD / YYYYMMDD from the file path
and get_or_create() a RehearsalSession. Both the watcher nc-upload
endpoint and the nc-scan endpoint now set song.session_id when a
dated folder is detected. Existing songs without a session_id are
back-filled on the next import of the same folder.

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

api/src/rehearsalhub/routers/internal.py

@@ -10,8 +10,10 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from rehearsalhub.db.engine import get_session
 from rehearsalhub.repositories.audio_version import AudioVersionRepository
 from rehearsalhub.repositories.band import BandRepository
+from rehearsalhub.repositories.rehearsal_session import RehearsalSessionRepository
 from rehearsalhub.repositories.song import SongRepository
 from rehearsalhub.schemas.audio_version import AudioVersionCreate
+from rehearsalhub.services.session import parse_rehearsal_date
 from rehearsalhub.services.song import SongService
 
 log = logging.getLogger(__name__)
@@ -69,6 +71,15 @@ async def nc_upload(
     if event.nc_file_etag and await version_repo.get_by_etag(event.nc_file_etag):
         return {"status": "skipped", "reason": "version already registered"}
 
+    # Resolve or create rehearsal session from YYMMDD folder segment
+    session_repo = RehearsalSessionRepository(session)
+    rehearsal_date = parse_rehearsal_date(path)
+    rehearsal_session_id = None
+    if rehearsal_date:
+        rehearsal_session = await session_repo.get_or_create(band.id, rehearsal_date, nc_folder)
+        rehearsal_session_id = rehearsal_session.id
+        log.debug("nc-upload: linked to session %s (%s)", rehearsal_session_id, rehearsal_date)
+
     song_repo = SongRepository(session)
     song = await song_repo.get_by_nc_folder_path(nc_folder)
     if song is None:
@@ -76,6 +87,7 @@ async def nc_upload(
     if song is None:
         song = await song_repo.create(
             band_id=band.id,
+            session_id=rehearsal_session_id,
             title=title,
             status="jam",
             notes=None,
@@ -83,6 +95,8 @@ async def nc_upload(
             created_by=None,
         )
         log.info("nc-upload: created song '%s' for band '%s'", title, band_slug)
+    elif rehearsal_session_id and song.session_id is None:
+        song = await song_repo.update(song, session_id=rehearsal_session_id)
 
     # Use first member of the band as uploader (best-effort for watcher uploads)
     from sqlalchemy import select