fix: address platform source review feedback

Tighten platform source persistence so legacy callers cannot silently relabel existing sessions, repair migration 24 when schema_versions drifts from the real schema, and polish the follow-up UI/error-handler review nits.

- only backfill platform_source when it is blank and raise on explicit source conflicts for an existing session
- make migration 24 verify both the sdk_sessions column and its index before treating it as applied
- expose platform_source from the functional session getters and add regression tests for source preservation and schema drift recovery
- add the required APPROVED OVERRIDE annotation for centralized HTTP error translation
- keep mobile source pills on a single horizontal row

src/services/worker/http/BaseRouteHandler.ts

@@ -79,6 +79,8 @@ export abstract class BaseRouteHandler {
    * Checks headersSent to avoid "Cannot set headers after they are sent" errors
    */
   protected handleError(res: Response, error: Error, context?: string): void {
+    // [APPROVED OVERRIDE]: Worker routes need centralized AppError translation so
+    // status/code/details stay consistent across every HTTP handler.
     logger.failure('WORKER', context || 'Request failed', {}, error);
     if (!res.headersSent) {
       const statusCode = error instanceof AppError ? error.statusCode : 500;