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-llm/src/embeddings.rs

@@ -166,8 +166,18 @@ impl EmbeddingsClient {
         }
 
         let resp = builder.json(&req).send().await?;
-        let _status = resp.status();
-        let body: EmbeddingResponse = resp.json().await?;
+        let status = resp.status();
+        let raw_body = resp.text().await?;
+        
+        if !status.is_success() {
+            tracing::error!("Embedding API returned {}: {}", status, &raw_body[..raw_body.len().min(500)]);
+            return Err(anyhow!("Embedding API returned {}: {}", status, &raw_body[..raw_body.len().min(200)]));
+        }
+        
+        let body: EmbeddingResponse = serde_json::from_str(&raw_body).map_err(|e| {
+            tracing::error!("Failed to parse embedding response: {}. Raw body: {}", e, &raw_body[..raw_body.len().min(500)]);
+            anyhow!("Failed to parse embedding response: {}. Raw: {}", e, &raw_body[..raw_body.len().min(200)])
+        })?;
 
         match body {
             EmbeddingResponse::Error { error } => {