fix(observability): fix OCR target name + add Ollama scrape job (#737)

- prometheus.yml: ocr:8000 → ocr-service:8000 (Docker service name is
  ocr-service, not ocr — current scrape target has never resolved)
- Add Ollama scrape job on ollama:11434 /metrics

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/observability/prometheus/prometheus.yml

@@ -20,4 +20,10 @@ scrape_configs:
   - job_name: ocr-service
     metrics_path: /metrics
     static_configs:
-      - targets: ['ocr:8000']
+      - targets: ['ocr-service:8000']
+
+  - job_name: ollama
+    metrics_path: /metrics
+    static_configs:
+      # Uses the Docker service name for reliable DNS resolution.
+      - targets: ['ollama:11434']