fix: scan visibility, NC folder validation, watcher logging

- nc-scan: detailed INFO logging of every path found, subfolder
  contents and skip reasons; 502 now includes the exact folder and
  error so user sees a real message instead of a blank result
- band creation: if nc_base_path is explicitly given, verify the
  folder exists in Nextcloud before saving — returns 422 with a
  clear message to the user; auto-generated paths still do MKCOL
- songs search: add ?unattributed=true to return songs with no
  session_id (files not in a YYMMDD folder)
- BandPage: show "Unattributed Recordings" section below sessions
  so scanned files without a dated folder always appear
- watcher event_loop: promote all per-activity log lines from DEBUG
  to INFO so they're visible in default Docker Compose log output;
  log normalized path and skip reason for every activity

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

api/src/rehearsalhub/repositories/song.py

@@ -51,6 +51,7 @@ class SongRepository(BaseRepository[Song]):
         bpm_min: float | None = None,
         bpm_max: float | None = None,
         session_id: uuid.UUID | None = None,
+        unattributed: bool = False,
     ) -> list[Song]:
         from sqlalchemy import cast, func
         from sqlalchemy.dialects.postgresql import ARRAY
@@ -75,6 +76,8 @@ class SongRepository(BaseRepository[Song]):
             stmt = stmt.where(Song.global_bpm <= bpm_max)
         if session_id is not None:
             stmt = stmt.where(Song.session_id == session_id)
+        if unattributed:
+            stmt = stmt.where(Song.session_id.is_(None))
 
         result = await self.session.execute(stmt)
         return list(result.scalars().all())