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/meta/_journal.json

@@ -57,6 +57,13 @@
       "when": 1774116777513,
       "tag": "0007_equal_nekra",
       "breakpoints": true
+    },
+    {
+      "idx": 8,
+      "version": "7",
+      "when": 1774123280818,
+      "tag": "0008_windy_cloak",
+      "breakpoints": true
     }
   ]
 }