feat(auth): @mosaic/auth — BetterAuth email/password setup

Implements P0-004: authentication package with BetterAuth + Drizzle adapter.

- BetterAuth configured with Drizzle adapter (usePlural for table mapping)
- Email/password authentication enabled
- Role field (admin/member/viewer) as additional user field
- 7-day sessions with daily refresh
- Mounted at /api/auth/* basePath
- createAuth() factory takes Db instance for DI flexibility
- BETTER_AUTH_SECRET and BETTER_AUTH_URL env vars added to .env.example

Verified: auth instance created, handler and API methods confirmed.

Closes #4

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

.env.example

@@ -12,5 +12,9 @@ VALKEY_URL=redis://localhost:6380
 OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318
 OTEL_SERVICE_NAME=mosaic-gateway
 
+# Auth (BetterAuth)
+BETTER_AUTH_SECRET=change-me-to-a-random-32-char-string
+BETTER_AUTH_URL=http://localhost:4000
+
 # Gateway
 GATEWAY_PORT=4000