refactor: config-driven libraries; drop folder-add UI

The frontend AddSourceFolderDialog let users register source roots
from inside the app, but with the bootstrap auto-creating one for
the /photos mount on first boot, the dialog was redundant in the
common case and confusing in every other (users had to know which
container path corresponded to their host directory). Going
config-driven matches Plex/Photoprism/Immich and matches the
mental model "the docker mount IS the library".

Frontend
- Deleted components/dialogs/AddSourceFolderDialog.tsx entirely.
- LeftSidebar drops the "+ Add Source Folder" button + bottom-bar
  layout, the addFolderMutation, the dead Plus action button on
  the (no-longer-existing) folders/heaps tree headers, and the
  Plus icon import.
- api.ts: removed sourceFolders.add(), library.browse(), and the
  BrowseChild / BrowseResponse types. The remaining sourceFolders
  surface is read-only (list + manual scan).
- LeftSidebar bottom strip is now just the "Scan all folders"
  button when there's at least one source root.

Backend
- Dropped POST /folders (no consumers) along with FolderCreate /
  FolderResponse pydantic models. The folders router header now
  documents the config-driven approach.
- Dropped GET /library/browse (no consumers). Removed the unused
  os/HTTPException/SourceRoot imports it brought in.
- cleanup_data_integrity now also walks the source roots and logs
  a warning for any whose path is missing on disk. Doesn't auto-
  delete (a missing path could be a temporarily unmounted drive)
  but surfaces enough hint to fix it. Returns the count in the
  summary dict alongside merged-duplicates.

Docs
- README "How libraries are managed" section rewritten to spell
  out that mounts ARE source roots, edit .env + restart, no UI for
  managing source roots. New "Changing or adding libraries"
  section walks through the typical edit-restart loop including
  the optional volume-nuke for a clean slate.
- "Adding more libraries" subsection covers multi-mount via
  edited compose with a note that auto-registration is roadmap.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/cleanup.py

@@ -119,6 +119,26 @@ async def _recompute_folder_counts(session: AsyncSession) -> None:
         f.photo_count = int(count_result.scalar() or 0)
 
 
+async def _warn_stale_source_roots(session: AsyncSession) -> int:
+    """Log a warning for any active source root whose path no longer exists
+    on disk. Doesn't delete — a missing path could be a temporarily
+    unmounted drive, and silently dropping user data is worse than
+    surfacing a noisy log line.
+    """
+    result = await session.execute(select(SourceRoot))
+    rows = result.scalars().all()
+    stale = 0
+    for sr in rows:
+        if not os.path.isdir(sr.path):
+            stale += 1
+            logger.warning(
+                f"Source root '{sr.name}' path is missing on disk: {sr.path} "
+                f"— is the docker mount still in place? "
+                f"(Edit docker-compose.yml or PHOTO_DIRS in .env to fix.)"
+            )
+    return stale
+
+
 async def cleanup_data_integrity() -> dict:
     """Top-level entry point. Runs the dedupe + count refresh in a single
     transaction. Returns a small summary dict for logging."""
@@ -127,10 +147,12 @@ async def cleanup_data_integrity() -> dict:
             sr_deleted = await _dedupe_source_roots(session)
             f_deleted = await _dedupe_folders(session)
             await _recompute_folder_counts(session)
+            stale = await _warn_stale_source_roots(session)
             await session.commit()
             summary = {
                 "source_roots_merged": sr_deleted,
                 "folders_merged": f_deleted,
+                "source_roots_stale": stale,
             }
             if sr_deleted or f_deleted:
                 logger.info(f"Cleanup merged duplicates: {summary}")