feat: persist in-app notifications in Postgres (durable, cross-instance) #43

Problem

inAppHandler in packages/notifications keeps a module-level in-memory array capped at 1,000 messages. Notifications are lost on restart, invisible across gateway instances, and only reachable via package-local helpers — the dashboard has no read/read-mark API. The audit (L8) flags this as a durability/scale gap.

Scope

• Add an InAppNotification model to packages/database/prisma/schema.prisma (providerId, event, data JSON, read, createdAt) + migration
• Rewrite inAppHandler to persist to Postgres when DATABASE_URL is available (keep in-memory fallback for tests/no-DB)
• Gateway endpoints: GET /api/v1/notifications (paginated, wallet-scoped) and POST /api/v1/notifications/:id/read
• Dashboard badge showing unread count

Files to Modify

Acceptance Criteria

• Notifications survive a gateway restart
• Read/unread state is per-provider and wallet-scoped
• Pagination works (limit 50 default)
• In-memory fallback preserved when no DB
• Migration applies cleanly (prisma migrate dev)
• Unit tests for the DB handler + endpoints
• pnpm exec nx test gateway passes