feat: complete observability stack (O1-O13) (#52)

## Complete Observability Stack (O1-O13)

Implements all 13 observability issues in a single PR. 119 metrics total.

### Commits (one per issue)

| Issue | Title | Metrics |
|-------|-------|---------|
| **O10** | Prometheus metrics module + /metrics endpoint | Foundation |
| **O1** | Instrument ingest handler | I1-I12 (12) |
| **O2** | Instrument query handler | Q1-Q12 (12) |
| **O3** | Instrument context endpoint | C1-C8 (8) |
| **O4** | Relevance judge | R1-R9 (9) |
| **O5** | Write volume + storage metrics | W1-W12 (12) |
| **O6** | Pod resource observability | P1-P13 |
| **O7** | Availability + dependency health | A1-A10 (10) |
| **O8** | Ingest rate pattern tracking | IR1-IR10 (10) |
| **O9** | Postgres internal observability | PG1-PG33 |
| **O11** | Grafana dashboard | 12 panels |
| **O12** | Prometheus alerting rules | 11 alerts |
| **O13** | Relevance evaluation CronJob | K8s manifest |

### Key Changes

- **metrics.rs**: Zero-dependency Prometheus metrics (Counter, Gauge, Histogram, Timer)
- **GET /metrics**: Prometheus text exposition format endpoint
- **Ingest/Query/Context handlers**: Instrumented with latency, errors, auth failures
- **Health check**: DB dependency check with latency tracking
- **Background task**: Periodic DB stats collection (entity/edge counts, pool stats)
- **Relevance judge**: Threshold-based eval with precision/recall/F1 tracking
- **Grafana dashboard**: 12 panels covering all metric groups
- **Alert rules**: 11 PrometheusRule alerts (availability, latency, errors, quality)
- **CronJob**: Periodic relevance evaluation with sample queries

### Testing

- 506 tests passing (0 failures)
- All metrics modules have unit tests
- Relevance judge: 4 tests

### Deploy

```bash
# Grafana dashboard
kubectl apply -f k8s/infra/grafana-dashboard.json

# Prometheus alerts
kubectl apply -f k8s/infra/prometheus-alerts.yaml

# Relevance eval CronJob
kubectl apply -f k8s/infra/relevance-eval-cronjob.yaml
```

Closes #27 #28 #29 #30 #31 #32 #33 #34 #35 #36 #37 #38 #39

---------

Co-authored-by: rock <[email protected]>
Reviewed-on: #52
Co-authored-by: poimen <[email protected]>

crates/mem-cli/src/handlers/middleware.rs

@@ -61,6 +61,49 @@ pub fn validate_and_rate_limit(
     Ok(())
 }
 
+/// Extract user identity from JWT claims (sub field)
+///
+/// Tries to decode JWT from Authorization header to get `sub` claim.
+/// Falls back to "anonymous" if auth is disabled or header missing.
+/// Used by metrics to track errors/requests per user.
+pub fn extract_user_id(req: &HttpRequest, state: &AppState) -> String {
+    // If auth disabled, check synthetic claims
+    if state.jwt_validator.is_none() {
+        return "anonymous".to_string();
+    }
+
+    // Try to extract sub from JWT
+    let token = req.headers()
+        .get("Authorization")
+        .and_then(|h| h.to_str().ok())
+        .and_then(|h| h.strip_prefix("Bearer "))
+        .unwrap_or("");
+
+    if token.is_empty() {
+        return "anonymous".to_string();
+    }
+
+    // Decode JWT payload without validation (already validated by validate_and_rate_limit)
+    // JWT format: header.payload.signature
+    let parts: Vec<&str> = token.split('.').collect();
+    if parts.len() != 3 {
+        return "anonymous".to_string();
+    }
+
+    // Decode base64 payload
+    use base64::Engine;
+    let engine = base64::engine::general_purpose::URL_SAFE_NO_PAD;
+    if let Ok(payload_bytes) = engine.decode(parts[1]) {
+        if let Ok(payload) = serde_json::from_slice::<serde_json::Value>(&payload_bytes) {
+            if let Some(sub) = payload.get("sub").and_then(|s| s.as_str()) {
+                return sub.to_string();
+            }
+        }
+    }
+
+    "anonymous".to_string()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;