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/db/repos/requestDetailsRepo.js

@@ -174,6 +174,12 @@ export async function getRequestDetails(filter = {}) {
   };
 }
 
+export async function getDistinctProviders() {
+  const db = await getAdapter();
+  const rows = db.all(`SELECT DISTINCT provider FROM requestDetails WHERE provider IS NOT NULL ORDER BY provider ASC`);
+  return rows.map((r) => r.provider);
+}
+
 export async function getRequestDetailById(id) {
   const db = await getAdapter();
   const row = db.get(`SELECT data FROM requestDetails WHERE id = ?`, [id]);