feat(db): @mosaic/db — Drizzle schema, PG connection, migrations

Implements P0-003: shared database package with Drizzle ORM.

Schema: 13 tables across 3 domains:
- Auth (BetterAuth-compatible): users, sessions, accounts, verifications
- Brain: projects, missions, tasks, events, agents, tickets, appreciations
- Chat: conversations, messages

Features:
- createDb() factory with pool cleanup via DbHandle.close()
- runMigrations() with try/finally for connection safety
- drizzle-kit config for generate/push/migrate/studio
- Full FK relationships and indexes on query-hot columns
- ESM package with NodeNext resolution

Verified: schema pushed to PG 17, CRUD round-trip tested.

Closes #3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/package.json

@@ -1,6 +1,7 @@
 {
   "name": "@mosaic/db",
   "version": "0.0.0",
+  "type": "module",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
   "exports": {
@@ -13,10 +14,21 @@
     "build": "tsc",
     "lint": "eslint src",
     "typecheck": "tsc --noEmit",
-    "test": "vitest run"
+    "test": "vitest run",
+    "db:generate": "drizzle-kit generate",
+    "db:migrate": "drizzle-kit migrate",
+    "db:push": "drizzle-kit push",
+    "db:studio": "drizzle-kit studio"
   },
   "devDependencies": {
+    "@types/node": "^22.0.0",
+    "drizzle-kit": "^0.31.9",
+    "tsx": "^4.0.0",
     "typescript": "^5.8.0",
     "vitest": "^2.0.0"
+  },
+  "dependencies": {
+    "drizzle-orm": "^0.45.1",
+    "postgres": "^3.4.8"
   }
 }