fix: DB backup on schema change, MCP child cleanup, codex models, usage providers OOM

- Backup DB only on real SCHEMA_VERSION change, not every app version bump
- Kill idle MCP stdio bridge children to prevent orphan process leaks
- Add getDistinctProviders to avoid loading every row JSON blob (OOM fix)
- Update codex model list (gpt-5.6 sol/terra/luna, drop 5.3 codex variants)
- Reorder Claude default models (fable first)

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lib/requestDetailsDb.js

@@ -1,4 +1,4 @@
 // Shim → re-export from new SQLite-based DB layer (src/lib/db/)
 export {
-  saveRequestDetail, getRequestDetails, getRequestDetailById,
+  saveRequestDetail, getRequestDetails, getRequestDetailById, getDistinctProviders,
 } from "@/lib/db/index.js";