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/config.py

@@ -27,22 +27,7 @@ class AuthConfig(BaseSettings):
 class StorageConfig(BaseSettings):
     """Storage configuration."""
 
-    devices_file: str = "data/devices.json"
-    templates_file: str = "data/capture_templates.json"
-    postprocessing_templates_file: str = "data/postprocessing_templates.json"
-    picture_sources_file: str = "data/picture_sources.json"
-    output_targets_file: str = "data/output_targets.json"
-    pattern_templates_file: str = "data/pattern_templates.json"
-    color_strip_sources_file: str = "data/color_strip_sources.json"
-    audio_sources_file: str = "data/audio_sources.json"
-    audio_templates_file: str = "data/audio_templates.json"
-    value_sources_file: str = "data/value_sources.json"
-    automations_file: str = "data/automations.json"
-    scene_presets_file: str = "data/scene_presets.json"
-    color_strip_processing_templates_file: str = "data/color_strip_processing_templates.json"
-    sync_clocks_file: str = "data/sync_clocks.json"
-    gradients_file: str = "data/gradients.json"
-    weather_sources_file: str = "data/weather_sources.json"
+    database_file: str = "data/ledgrab.db"
 
 
 class MQTTConfig(BaseSettings):