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/config/demo_config.yaml

@@ -19,12 +19,7 @@ auth:
     demo: "demo"
 
 storage:
-  devices_file: "data/devices.json"
-  templates_file: "data/capture_templates.json"
-  postprocessing_templates_file: "data/postprocessing_templates.json"
-  picture_sources_file: "data/picture_sources.json"
-  output_targets_file: "data/output_targets.json"
-  pattern_templates_file: "data/pattern_templates.json"
+  database_file: "data/ledgrab.db"
 
 mqtt:
   enabled: false