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/src/lib/db/schema.ts

@@ -42,6 +42,7 @@ export const users = pgTable("users", {
   email: text("email").notNull().unique(),
   passwordHash: text("password_hash").notNull(),
   createdAt: timestamp("created_at").defaultNow().notNull(),
+  agentSignatureData: text("agent_signature_data"),
 });
 
 export const documentStatusEnum = pgEnum("document_status", [