feat(03-02): implement social draft pipeline backend

- Add social_post to ApprovalType union in types.ts
- Add social_post to approvalInputSchema enum in contracts.ts
- Add POST /api/companies/:companyId/social/draft endpoint in api-server.ts
- Add social_post handling in handleApprovalResolved in automation-service.ts
- Browser action payloadJson includes scheduledAt for startup recovery (03-03 contract)
- Scheduled posts use setTimeout for deferred dispatch
- All 13 unit tests pass

src/shared/types.ts

@@ -64,7 +64,8 @@ export type ApprovalType =
   | "vendor_approval"
   | "document_review"
   | "production_deploy"
-  | "incident_escalation";
+  | "incident_escalation"
+  | "social_post";
 export type RunStatus =
   | "queued"
   | "running"