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/services/band.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import logging
 import uuid
 
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -9,22 +10,42 @@ from rehearsalhub.repositories.band import BandRepository
 from rehearsalhub.schemas.band import BandCreate, BandReadWithMembers
 from rehearsalhub.storage.nextcloud import NextcloudClient
 
+log = logging.getLogger(__name__)
+
 
 class BandService:
     def __init__(self, session: AsyncSession, storage: NextcloudClient | None = None) -> None:
         self._repo = BandRepository(session)
         self._storage = storage or NextcloudClient()
 
-    async def create_band(self, data: BandCreate, creator_id: uuid.UUID, creator: object | None = None) -> Band:
+    async def create_band(
+        self,
+        data: BandCreate,
+        creator_id: uuid.UUID,
+        creator: object | None = None,
+    ) -> Band:
         if await self._repo.get_by_slug(data.slug):
             raise ValueError(f"Slug already taken: {data.slug}")
 
         nc_folder = (data.nc_base_path or f"bands/{data.slug}/").strip("/") + "/"
         storage = NextcloudClient.for_member(creator) if creator else self._storage
-        try:
-            await storage.create_folder(nc_folder)
-        except Exception:
-            pass  # NC might not be reachable during tests; folder creation is best-effort
+
+        if data.nc_base_path:
+            # User explicitly specified a folder — verify it actually exists in NC.
+            log.info("Checking NC folder existence: %s", nc_folder)
+            try:
+                await storage.get_file_metadata(nc_folder.rstrip("/"))
+            except Exception as exc:
+                log.warning("NC folder '%s' not accessible: %s", nc_folder, exc)
+                raise LookupError(f"Nextcloud folder '{nc_folder}' not found or not accessible")
+        else:
+            # Auto-generated path — create it (idempotent MKCOL).
+            log.info("Creating NC folder: %s", nc_folder)
+            try:
+                await storage.create_folder(nc_folder)
+            except Exception as exc:
+                # Not fatal — NC may be temporarily unreachable during dev/test.
+                log.warning("Could not create NC folder '%s': %s", nc_folder, exc)
 
         band = await self._repo.create(
             name=data.name,
@@ -33,6 +54,7 @@ class BandService:
             nc_folder_path=nc_folder,
         )
         await self._repo.add_member(band.id, creator_id, role="admin")
+        log.info("Created band '%s' (slug=%s, nc_folder=%s)", data.name, data.slug, nc_folder)
         return band
 
     async def get_band_with_members(self, band_id: uuid.UUID) -> Band | None: