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/drizzle/meta/_journal.json

@@ -0,0 +1,13 @@
+{
+  "version": "7",
+  "dialect": "postgresql",
+  "entries": [
+    {
+      "idx": 0,
+      "version": "7",
+      "when": 1773368153122,
+      "tag": "0000_loud_ezekiel_stane",
+      "breakpoints": true
+    }
+  ]
+}