feat(db): add structured logging for database connection pool events …

[extolkom]

Summary

Closes #764

Database connection pool exhaustion was previously invisible until requests
started timing out. This adds structured debug-level logging on pool acquire
and release events, plus a warn-level log when acquisition wait time exceeds
1000ms, so operators can monitor pool health proactively.

Changes

• Added src/utils/db-pool-log.utils.ts with logDbPoolAcquire and
  logDbPoolRelease helpers
• Wired both into the Prisma client query extension middleware in
  src/utils/prisma.utils.ts
• Added unit tests in src/utils/db-pool-log.utils.test.ts (10 cases),
  including a boundary test at exactly 1000ms vs 1001ms

Acceptance Criteria

• Debug log on acquire with all four fields (pool_size, idle_count,
  wait_time_ms, acquired_at)
• Debug log on release with all three fields (pool_size, idle_count,
  held_for_ms)
• Warn log when wait time exceeds 1000ms
• Log level is debug for normal events

Testing

• pnpm test src/utils/db-pool-log.utils.test.ts — 10/10 passing
• pnpm check (eslint + tsc) — 0 errors
• npx prettier --check — passing
• Branch rebased cleanly onto latest origin/main, no conflicts

Notes for reviewers

Field naming follows the repo's existing snake_case convention used
elsewhere in the logging utils. No new logging dependency was introduced —
this reuses the existing logger already in use in prisma.utils.ts.

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