feat(backend): API & Database Scaling Part 41 (#286)

[Joeloo1]

Summary

• GET /api/v1/db-scaling/bgwriter-stats — single-row snapshot from pg_stat_bgwriter covering checkpoint frequency (timed vs requested), buffer write counts per writer, backend fsync calls, and checkpoint I/O durations
• GET /api/v1/db-scaling/database-stats — snapshot from pg_stat_database for the active DB: transaction throughput, buffer cache hit ratio, temporary file usage, deadlock count, and conflict count
• WebhookService — event dispatch with exponential-backoff retry (up to 4 attempts) and delivery log writes to webhook_delivery_logs
• OpenAPI spec updated for both new endpoints

Acceptance Criteria

• API endpoints are functional and tested
• Proper error responses (5xx via next(err)) are implemented
• No schema changes — both endpoints query built-in Postgres system views; no migration required
• Integration tests included: happy path, zero-row fallback, and 500-on-service-error for each endpoint

Closes #731

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