devops(observability): add Prometheus + Node Exporter + cAdvisor for host and container metrics

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

infra/observability/prometheus/prometheus.yml

@@ -0,0 +1,28 @@
+global:
+  scrape_interval: 15s
+  evaluation_interval: 15s
+
+scrape_configs:
+  - job_name: node
+    static_configs:
+      - targets: ['node-exporter:9100']
+
+  - job_name: cadvisor
+    static_configs:
+      - targets: ['cadvisor:8080']
+
+  - job_name: spring-boot
+    metrics_path: /actuator/prometheus
+    static_configs:
+      # Uses the Docker service name (not container_name) for reliable DNS resolution.
+      # Target will show as DOWN until backend instrumentation issue adds
+      # micrometer-registry-prometheus and exposes the endpoint — this is expected.
+      - targets: ['backend:8080']
+
+  - job_name: ocr-service
+    metrics_path: /metrics
+    static_configs:
+      # TODO: remove or add prometheus-client to ocr-service.
+      # The Python OCR service does not currently expose Prometheus metrics.
+      # This target will show as DOWN until prometheus-client is added to ocr-service.
+      - targets: ['ocr:8000']