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/routers/bands.py

@@ -34,6 +34,8 @@ async def create_band(
         band = await svc.create_band(data, current_member.id, creator=current_member)
     except ValueError as e:
         raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail=str(e))
+    except LookupError as e:
+        raise HTTPException(status_code=status.HTTP_422_UNPROCESSABLE_ENTITY, detail=str(e))
     return BandRead.model_validate(band)