feat(monitoring): add LLM Grafana dashboard, Prometheus scraping, and alerts

Wire mana-llm service into the monitoring stack:

Prometheus (docker/prometheus/prometheus.yml):
- Add mana-llm scrape job (port 3025, 15s interval)
- Include mana-llm in ServiceDown alert expression

Alerts (docker/prometheus/alerts.yml):
- New llm_alerts group with 4 rules:
  - LLMServiceDown: mana-llm down > 1 min (critical)
  - LLMHighErrorRate: > 10% errors for 5 min (warning)
  - OllamaProviderDown: > 50% requests via Google fallback (warning)
  - LLMSlowResponses: p95 > 30s for 5 min (warning)

Grafana Dashboard (docker/grafana/dashboards/mana-llm.json):
- 6 stat panels: status, req/min, error rate, fallback rate, latency, tokens/min
- Requests by Provider (stacked area: Ollama vs Google vs OpenRouter)
- Tokens by Type (prompt vs completion)
- Latency Percentiles (p50, p90, p99)
- Latency by Provider comparison
- Requests by Model breakdown
- Errors by Type
- Google Fallback Rate over time (with threshold coloring)
- Provider Distribution pie chart (24h)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/prometheus/prometheus.yml

@@ -158,6 +158,13 @@ scrape_configs:
   # Core Services
   # ============================================
 
+  # Mana LLM Gateway (Ollama + Google Fallback)
+  - job_name: 'mana-llm'
+    static_configs:
+      - targets: ['mana-llm:3025']
+    metrics_path: '/metrics'
+    scrape_interval: 15s
+
   # Mana Search Service
   - job_name: 'mana-search'
     static_configs: