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/songs.py

@@ -60,6 +60,7 @@ async def search_songs(
     bpm_min: float | None = Query(None, ge=0),
     bpm_max: float | None = Query(None, ge=0),
     session_id: uuid.UUID | None = Query(None),
+    unattributed: bool = Query(False, description="Only songs with no rehearsal session"),
     session: AsyncSession = Depends(get_session),
     current_member: Member = Depends(get_current_member),
 ):
@@ -78,6 +79,7 @@ async def search_songs(
         bpm_min=bpm_min,
         bpm_max=bpm_max,
         session_id=session_id,
+        unattributed=unattributed,
     )
     return [
         SongRead.model_validate(s, update={"version_count": len(s.versions)})
@@ -173,14 +175,20 @@ async def scan_nextcloud(
     skipped_count = 0
     band_folder = band.nc_folder_path or f"bands/{band.slug}/"
 
-    log.info("Starting NC scan for band '%s' in folder '%s'", band.slug, band_folder)
+    log.info("NC scan START — band='%s' folder='%s' nc_user='%s'", band.slug, band_folder, nc._auth[0])
 
     try:
         items = await nc.list_folder(band_folder)
     except Exception as exc:
-        raise HTTPException(status_code=status.HTTP_502_BAD_GATEWAY, detail=f"Nextcloud unreachable: {exc}")
+        log.error("NC scan FAILED — could not list '%s': %s", band_folder, exc)
+        raise HTTPException(
+            status_code=status.HTTP_502_BAD_GATEWAY,
+            detail=f"Cannot read Nextcloud folder '{band_folder}': {exc}",
+        )
 
-    log.info("Found %d top-level entries in '%s'", len(items), band_folder)
+    log.info("NC scan — found %d top-level entries in '%s'", len(items), band_folder)
+    for item in items:
+        log.info("  entry href=%s → rel=%s", item.path, relative(item.path))
 
     # Collect (nc_file_path, nc_folder, song_title, rehearsal_label) tuples.
     # nc_folder is the directory that groups versions of the same song.
@@ -195,27 +203,39 @@ async def scan_nextcloud(
             try:
                 sub_items = await nc.list_folder(rel)
             except Exception as exc:
-                log.warning("Could not list subfolder '%s': %s", rel, exc)
+                log.warning("NC scan — could not list subfolder '%s': %s", rel, exc)
                 continue
 
+            all_sub = [relative(s.path) for s in sub_items]
             audio_files = [s for s in sub_items if Path(relative(s.path)).suffix.lower() in AUDIO_EXTENSIONS]
-            log.info("Subfolder '%s': %d audio files found", dir_name, len(audio_files))
+            log.info(
+                "NC scan — subfolder '%s': %d entries total, %d audio files",
+                dir_name, len(all_sub), len(audio_files),
+            )
+            for s in sub_items:
+                sr = relative(s.path)
+                ext = Path(sr).suffix.lower()
+                if ext and ext not in AUDIO_EXTENSIONS:
+                    log.info("    skip (not audio ext=%s): %s", ext, sr)
 
             for sub in audio_files:
                 sub_rel = relative(sub.path)
                 song_title = Path(sub_rel).stem
-                # Each file in a rehearsal folder is its own song,
-                # grouped under its own sub-subfolder path for version tracking.
                 song_folder = str(Path(sub_rel).parent) + "/"
-                rehearsal_label = dir_name  # e.g. "231015" or "2023-10-15"
+                rehearsal_label = dir_name
+                log.info("    queue for import: %s → title='%s' folder='%s'", sub_rel, song_title, song_folder)
                 to_import.append((sub_rel, song_folder, song_title, rehearsal_label))
         else:
-            if Path(rel).suffix.lower() in AUDIO_EXTENSIONS:
+            ext = Path(rel).suffix.lower()
+            if ext in AUDIO_EXTENSIONS:
                 folder = str(Path(rel).parent) + "/"
                 title = Path(rel).stem
+                log.info("  queue for import (root-level): %s → title='%s'", rel, title)
                 to_import.append((rel, folder, title, None))
+            elif ext:
+                log.info("  skip root-level (not audio ext=%s): %s", ext, rel)
 
-    log.info("NC scan: %d audio files to evaluate for import", len(to_import))
+    log.info("NC scan — %d audio files queued for import", len(to_import))
 
     song_repo = SongRepository(session)
     from rehearsalhub.schemas.audio_version import AudioVersionCreate  # noqa: PLC0415