T10: Monitoring, Performance, Documentation & Environment Config — 38 tests, ruff clean, docs OK

- Extended health endpoint: DB+Redis+Storage+Worker checks with degraded status
- Prometheus metrics endpoint: admin-only, text/plain format
- Metrics: http_requests_total, db_pool_connections, arq_jobs_total
- Structured JSON logging (structlog): timestamp, level, method, path, status, duration_ms, tenant_id
- Performance: page_size max 100 enforced (422), streaming CSV export (StreamingResponse)
- Scripts: seed_perf_data.py, check_indexes.py
- Docs: admin-guide.md (Deploy, Backup, Restore, Env-Vars, Troubleshooting), api-overview.md
- README updated: prod setup, API section, env profiles, admin-guide link
- .env.example: added SECRET_KEY, STORAGE_PATH, SMTP_* vars
- 38 new tests, full regression 564/564 pass (0 failures)
- Ruff: all checks passed

requirements.txt

@@ -29,3 +29,7 @@ aiofiles>=23.2
 jinja2>=3.1
 greenlet>=3.0
 openpyxl>=3.1
+
+# Monitoring
+prometheus-client>=0.20
+structlog>=24.0