fix: deduplicate scanner-created source_roots and folders

Two related fixes:

1. Prevention — scanner now normalizes paths before lookup/insert
   in get_or_create_source_root and get_or_create_folder. Trailing
   slashes, redundant separators, and `.` segments all collapse to
   the same row. _normalize_path uses os.path.normpath; symlinks
   are intentionally NOT resolved so mount paths stay intact for
   cross-machine portability.

2. Cleanup — new app/services/cleanup.py runs on backend startup
   (idempotent) and merges any pre-existing duplicates left over
   from older scanner versions:
   - Groups source_roots by normalized path. Picks the canonical
     row (preferring one with a non-empty name and the earliest
     added_at), re-points child Folder rows via UPDATE, and
     deletes the duplicates.
   - Same for folders, with photo_count as the tiebreaker. Photos
     get re-pointed to the canonical folder via UPDATE.
   - Recomputes folder.photo_count from the actual non-discarded
     photo membership so the sidebar count matches reality.

Wired into main.py's lifespan handler. On the dev DB this merged
the empty-name "/host/Pictures/MulitaTest/" duplicate that was
showing up alongside the canonical MulitaTest source root.

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

backend/app/main.py

@@ -13,6 +13,7 @@ from app.config import settings
 from app.database import init_db
 from app.routers import photos, folders, heaps, tags, discard, library
 from app.services.scanner import start_initial_scan
+from app.services.cleanup import cleanup_data_integrity
 
 # Configure logging
 logging.basicConfig(
@@ -25,17 +26,24 @@ logger = logging.getLogger(__name__)
 async def lifespan(app: FastAPI):
     """Manage application lifecycle"""
     logger.info("Starting Mulita application...")
-    
+
     # Initialize database
     await init_db()
-    
+
+    # One-shot cleanup of duplicate source_roots / folders left over from
+    # earlier scanner versions that didn't normalize paths. Idempotent.
+    try:
+        await cleanup_data_integrity()
+    except Exception as e:
+        logger.error(f"Startup cleanup failed (continuing): {e}")
+
     # Start initial scan if configured
     if settings.scanner.initial_scan_on_start:
         logger.info("Starting initial library scan...")
         await start_initial_scan()
-    
+
     yield
-    
+
     logger.info("Shutting down Mulita application...")
 
 # Create FastAPI app