Add database connection health check indicator

## Task
Implement the database health indicator with actual Prisma query.

## Details
- In `apps/api/src/health/indicators/database.health.ts`
- Inject PrismaService
- Execute `SELECT 1` via `prisma.$queryRaw`
- Return healthy only when query succeeds
- Log query errors for debugging

## Path
- `apps/api/src/health/indicators/database.health.ts`

## Test
Run `pnpm --filter api build` to verify.