devops(observability): add Tempo for distributed trace storage (OTLP receiver)

Closes #575

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

docker-compose.observability.yml

@@ -112,8 +112,29 @@ services:
         condition: service_healthy
 
   # --- Traces: Tempo ---
-  #   tempo: (see future issue)
-  #
+
+  tempo:
+    image: grafana/tempo:2.7.2
+    container_name: obs-tempo
+    restart: unless-stopped
+    volumes:
+      - ./infra/observability/tempo/tempo.yml:/etc/tempo.yml:ro
+      - tempo_data:/var/tempo
+    command: -config.file=/etc/tempo.yml
+    expose:
+      - "3200"   # Grafana queries Tempo on this port (obs-net only)
+      - "4317"   # OTLP gRPC — backend sends traces here (archiv-net)
+      - "4318"   # OTLP HTTP — alternative transport (archiv-net)
+    healthcheck:
+      test: ["CMD-SHELL", "wget -qO- http://localhost:3200/ready | grep -q ready || exit 1"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+      start_period: 15s
+    networks:
+      - archiv-net   # backend (archive-backend) reaches tempo:4317 over this network
+      - obs-net      # Grafana reaches tempo:3200 over this network
+
   # --- Dashboards: Grafana ---
   #   grafana: (see future issue)
   #