Migrate harness from in-memory stores to CloudNativePG

Replace all in-memory Map-backed stores (credentials, models, agents,
tasks, iterations, usage) with Drizzle ORM queries against the
homelab-pg PostgreSQL cluster. All store functions are now async.

- Add 6 harness_* tables to @homelab/db schema
- Generate and apply initial Drizzle migration
- Add lazy DB connection proxy to avoid build-time errors
- Wire DATABASE_URL from sealed secret into harness deployment
- Update all API routes, orchestrator, executor, and boot to await
  async store operations

apps/harness/package.json

@@ -10,7 +10,10 @@
     "test": "echo \"no tests yet\""
   },
   "dependencies": {
+    "@homelab/db": "workspace:^",
+    "drizzle-orm": "^0.36.0",
     "next": "^15.1.0",
+    "postgres": "^3.4.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "yaml": "^2.7.0"