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>

docs/scratchpads/mvp-20260312.md

@@ -43,6 +43,18 @@ User confirmed: start the planning gate.
 | 2       | 2026-03-13 | Vertical slice | P1-001, P1-007, P1-008, P2-001, P5-002, P6-005 | Communication spine built and merged (PR #61). Gateway + TUI + Discord. 3-agent gatekeeper review, 10/16 issues remediated, 4 deferred.                                                     |
 | 3       | 2026-03-13 | Foundation     | P0-002, P0-005, P0-006                         | Foundation layer merged (PR #65). Docker Compose (PG+pgvector, Valkey, OTEL Collector, Jaeger), OTEL auto-instrumentation in gateway, @mosaic/types with DTOs + Socket.IO typed event maps. |
 
+### Session 4 — Docker Compose fix
+
+| Session | Date       | Milestone  | Tasks Done | Outcome                                                                                                          |
+| ------- | ---------- | ---------- | ---------- | ---------------------------------------------------------------------------------------------------------------- |
+| 4       | 2026-03-12 | Foundation | (fix)      | Fixed Jaeger tag (2→2.6.0), remapped PG/Valkey ports (5433/6380) to avoid host conflicts. PR #66 merged to main. |
+
+**Verification evidence:**
+
+- All 4 containers healthy (PG, Valkey, OTEL Collector, Jaeger)
+- OTEL pipeline proven: `mosaic-gateway` service visible in Jaeger UI
+- Gateway traces flow through Collector → Jaeger
+
 ## Open Questions
 
 (none at this time)