feat: migrate storage from JSON files to SQLite

Replace 22 individual JSON store files with a single SQLite database
(data/ledgrab.db). All entity stores now use BaseSqliteStore backed by
SQLite with WAL mode, write-through caching, and thread-safe access.

- Add Database class with SQLite backup/restore API
- Add BaseSqliteStore as drop-in replacement for BaseJsonStore
- Convert all 16 entity stores to SQLite
- Move global settings (MQTT, external URL, auto-backup) to SQLite
  settings table
- Replace JSON backup/restore with SQLite snapshot backups (.db files)
- Remove partial export/import feature (backend + frontend)
- Update demo seed to write directly to SQLite
- Add "Backup Now" button to settings UI
- Remove StorageConfig file path fields (single database_file remains)

server/src/wled_controller/api/schemas/system.py

@@ -75,12 +75,9 @@ class PerformanceResponse(BaseModel):
 
 
 class RestoreResponse(BaseModel):
-    """Response after restoring configuration backup."""
+    """Response after restoring database backup."""
 
     status: str = Field(description="Status of restore operation")
-    stores_written: int = Field(description="Number of stores successfully written")
-    stores_total: int = Field(description="Total number of known stores")
-    missing_stores: List[str] = Field(default_factory=list, description="Store keys not found in backup")
     restart_scheduled: bool = Field(description="Whether server restart was scheduled")
     message: str = Field(description="Human-readable status message")