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

app/routes/metrics.py

@@ -0,0 +1,29 @@
+"""Prometheus metrics endpoint — admin-only."""
+
+from __future__ import annotations
+
+from fastapi import APIRouter, Depends
+from fastapi.responses import PlainTextResponse
+
+from app.core.monitoring import generate_metrics
+from app.deps import require_admin
+
+router = APIRouter(tags=["metrics"])
+
+
+@router.get(
+    "/api/v1/metrics",
+    response_class=PlainTextResponse,
+    dependencies=[Depends(require_admin)],
+)
+async def metrics():
+    """Prometheus metrics endpoint.
+
+    Returns Prometheus-formatted text/plain metrics.
+    Requires admin role — 403 for non-admin users.
+    """
+    data = generate_metrics()
+    return PlainTextResponse(
+        content=data.decode("utf-8"),
+        media_type="text/plain; version=0.0.4; charset=utf-8",
+    )