fix: remove destructive DELETE+INSERT shutdown save that caused progressive data loss

_save_all() in BaseSqliteStore did DELETE FROM table + INSERT all in-memory items
on every shutdown. Since SQLite stores use write-through caching (every CRUD writes
immediately), this was redundant. Worse, if in-memory state had fewer items than
the DB, the DELETE wiped rows and only partial data was reinserted.

- Make _save_all() a no-op (DB is always up to date via write-through)
- Replace self._save() with self._save_item() in 6 seed/default creation methods
- Remove _save_all_stores() shutdown hook (replaced with log-only message)

server/src/wled_controller/storage/gradient_store.py

@@ -58,7 +58,7 @@ class GradientStore(BaseSqliteStore[Gradient]):
         now = datetime.now(timezone.utc)
         for name, tuples in _BUILTIN_DEFS.items():
             gid = f"gr_builtin_{name}"
-            self._items[gid] = Gradient(
+            gradient = Gradient(
                 id=gid,
                 name=name.capitalize(),
                 stops=_tuples_to_stops(tuples),
@@ -67,7 +67,8 @@ class GradientStore(BaseSqliteStore[Gradient]):
                 updated_at=now,
                 description=f"Built-in {name} gradient",
             )
-        self._save()
+            self._items[gid] = gradient
+            self._save_item(gid, gradient)
         logger.info(f"Seeded {len(_BUILTIN_DEFS)} built-in gradients")
 
     # Aliases