feat(phase1): add image storage utilities

- Create backend/services/image_storage.py with 4 core functions:
  - sanitize_filename(): remove unsafe chars, limit to 255 chars, convert to lowercase
  - get_unique_filename(): handle collisions with UUID suffix (format: {name}_{uuid8}_{variant}.jpg)
  - ensure_image_directories(): create /images/ root and category subdirs on startup
  - save_image(): save bytes to /images/{category}/{filename}, returns relative path
- Create comprehensive test suite (22 tests) covering all functionality
- Integrate ensure_image_directories() into FastAPI startup event
- Directory structure: /images/{category}/{filename}
- Collision handling: auto-suffix with UUID if filename exists
- All tests passing, pathlib.Path for safe operations

backend/main.py

@@ -10,6 +10,7 @@ from .routers import items, operations, users, auth, sync, categories
 from .routers.admin import backups, ai_config, db_config
 from .logger import log
 from .scheduler import scheduler, sync_scheduler_config
+from .services.image_storage import ensure_image_directories
 
 # Create the database tables
 from .database import DATA_DIR, db_path
@@ -96,6 +97,8 @@ app.include_router(db_config.router)
 
 @app.on_event("startup")
 def startup_event():
+    log.info("[STARTUP] Initializing image storage directories...")
+    ensure_image_directories()
     log.info("[STARTUP] Starting background scheduler...")
     scheduler.start()
     sync_scheduler_config()