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>

eslint.config.mjs

@@ -4,7 +4,13 @@ import tsParser from '@typescript-eslint/parser';
 
 export default tseslint.config(
   {
-    ignores: ['**/dist/**', '**/node_modules/**', '**/.next/**', '**/coverage/**'],
+    ignores: [
+      '**/dist/**',
+      '**/node_modules/**',
+      '**/.next/**',
+      '**/coverage/**',
+      '**/drizzle.config.ts',
+    ],
   },
   {
     files: ['**/*.{ts,tsx}'],