feat(11-01): DB migration and API routes for agent signature storage

- Add agentSignatureData TEXT column to users table in schema.ts
- Generate migration 0008_windy_cloak.sql (ALTER TABLE users ADD COLUMN agent_signature_data text)
- Apply migration to local postgres database
- Create GET/PUT /api/agent/signature route with auth guard and input validation

teressa-copeland-homes/drizzle/0008_windy_cloak.sql

@@ -0,0 +1 @@
+ALTER TABLE "users" ADD COLUMN "agent_signature_data" text;