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/songs.py

@@ -12,10 +12,12 @@ from rehearsalhub.dependencies import get_current_member
 from rehearsalhub.repositories.audio_version import AudioVersionRepository
 from rehearsalhub.repositories.band import BandRepository
 from rehearsalhub.repositories.comment import CommentRepository
+from rehearsalhub.repositories.rehearsal_session import RehearsalSessionRepository
 from rehearsalhub.repositories.song import SongRepository
 from rehearsalhub.schemas.comment import SongCommentCreate, SongCommentRead
 from rehearsalhub.schemas.song import SongCreate, SongRead, SongUpdate
 from rehearsalhub.services.band import BandService
+from rehearsalhub.services.session import parse_rehearsal_date
 from rehearsalhub.services.song import SongService
 from rehearsalhub.storage.nextcloud import NextcloudClient
 
@@ -156,6 +158,7 @@ async def scan_nextcloud(
 
     nc = NextcloudClient.for_member(current_member)
     version_repo = AudioVersionRepository(session)
+    session_repo = RehearsalSessionRepository(session)
     song_svc = SongService(session)
 
     # dav_prefix to strip full WebDAV hrefs → user-relative paths
@@ -231,6 +234,13 @@ async def scan_nextcloud(
             skipped_count += 1
             continue
 
+        # Resolve rehearsal session from YYMMDD folder segment
+        rehearsal_date = parse_rehearsal_date(nc_file_path)
+        rehearsal_session_id = None
+        if rehearsal_date:
+            rs = await session_repo.get_or_create(band_id, rehearsal_date, nc_folder)
+            rehearsal_session_id = rs.id
+
         # Find or create song record
         song = await song_repo.get_by_nc_folder_path(nc_folder)
         if song is None:
@@ -239,14 +249,17 @@ async def scan_nextcloud(
             log.info("Creating new song '%s' (folder: %s)", song_title, nc_folder)
             song = await song_repo.create(
                 band_id=band_id,
+                session_id=rehearsal_session_id,
                 title=song_title,
                 status="jam",
-                notes=f"Rehearsal: {rehearsal_label}" if rehearsal_label else None,
+                notes=None,
                 nc_folder_path=nc_folder,
                 created_by=current_member.id,
             )
         else:
             log.info("Found existing song '%s' (id: %s)", song.title, song.id)
+            if rehearsal_session_id and song.session_id is None:
+                song = await song_repo.update(song, session_id=rehearsal_session_id)
 
         await song_svc.register_version(
             song.id,