fix: dynamic VALID_CATEGORIES from DB categories table

Same issue as canteen-asset-tracker — hardcoded category set was
{Furniture, Appliances, Utensils, Equipment, Other}, missing all
vending-machine categories (Bev, Snack, GF Bev, GF Food, etc.).

- admin_server.py: _load_categories() from DB at startup
- cantaloupe_sync.py: reads from categories table instead of
  hardcoded set (was silently downgrading GF Bev→Other on sync!)

admin_server.py

@@ -31,15 +31,26 @@ from cantaloupe_sync import router as cantaloupe_router, create_sync_tables
 
 # ─── Config ─────────────────────────────────────────────────────────────────
 
-VALID_CATEGORIES = {"Furniture", "Appliances", "Utensils & Serveware", "Equipment", "Other"}
-VALID_STATUSES = {"active", "maintenance", "retired"}
-VALID_ROLES = {"admin", "technician", "readonly"}
 DB_PATH = os.environ.get("CANTEEN_DB_PATH", str(Path(__file__).parent / "assets.db"))
 UPLOADS_DIR = Path(os.environ.get("CANTEEN_UPLOADS_DIR", str(Path(__file__).parent / ".." / "canteen-asset-tracker" / "uploads")))
 ICON_MAX_SIZE = 2 * 1024 * 1024       # 2 MB
 ICON_ALLOWED_EXTS = {".png", ".jpg", ".jpeg", ".svg"}
 PHOTO_ALLOWED_EXTS = {".png", ".jpg", ".jpeg"}
 
+def _load_categories() -> set:
+    """Load valid category names from the categories lookup table."""
+    try:
+        conn = sqlite3.connect(DB_PATH)
+        names = {r[0] for r in conn.execute("SELECT name FROM categories").fetchall()}
+        conn.close()
+        return names if names else {"Other"}
+    except Exception:
+        return {"Furniture", "Appliances", "Utensils & Serveware", "Equipment", "Other"}
+
+VALID_CATEGORIES = _load_categories()
+VALID_STATUSES = {"active", "maintenance", "retired"}
+VALID_ROLES = {"admin", "technician", "readonly"}
+
 
 # ─── Database ───────────────────────────────────────────────────────────────