feat: auto-create search/tts providers at startup with enable toggle

- Add `enable` column (default false) to search_providers and tts_providers tables
- Auto-create default entries for all provider types on startup (disabled by default)
- Add enable/disable Switch toggle in frontend for both search and TTS providers
- Show green status dot in sidebar for enabled providers, sort enabled first
- Filter bot settings dropdowns to only show enabled providers

db/migrations/0049_search_provider_enable.down.sql

@@ -0,0 +1,4 @@
+-- 0049_search_provider_enable (down)
+-- Remove the enable column from search_providers.
+
+ALTER TABLE search_providers DROP COLUMN IF EXISTS enable;