Expose a runnable backend without giving the ingress layer secrets

The backend can now be inspected through a loopback-only gateway while the database-aware API remains on the internal data network. A governed synthetic demo proves readiness, pgvector retrieval, reranking, and citation output through real HTTP without invoking cloud models.

Constraint: The previously exposed Bailian key is compromised and cannot be used for live validation

Constraint: The API must be locally reachable while retaining no internet egress

Rejected: Attach the API directly to the ingress network | a real socket test proved that configuration still had egress

Rejected: Publish a port from the internal-only network | Docker Desktop did not expose the host port

Confidence: high

Scope-risk: moderate

Reversibility: clean

Directive: Keep model and database credentials out of the gateway; do not relax the fixed demo identity/profile filters

Tested: make verify; 63 pytest tests; strict mypy; Ruff; Secret scan; Compose config; three backend image builds; API/DB/gateway healthy; migration exit 0; Swagger browser check; live/ready/meta/status/search HTTP; 20/20/20 index; API egress ENETUNREACH; empty gateway mounts and business environment

Not-tested: Live Bailian calls require a newly rotated key; full generated-answer flow and React UI are not implemented

compose.yaml

@@ -81,6 +81,76 @@ services:
       - data
     restart: "no"
 
+  api:
+    build:
+      context: ./backend
+    command: ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]
+    depends_on:
+      db:
+        condition: service_healthy
+      migrate:
+        condition: service_completed_successfully
+    environment: *runtime-config
+    secrets:
+      - postgres_app_password
+    networks:
+      - data
+    healthcheck:
+      test:
+        - CMD
+        - python
+        - -c
+        - >-
+          import urllib.request;
+          urllib.request.urlopen('http://127.0.0.1:8000/health/ready', timeout=2)
+      interval: 10s
+      timeout: 3s
+      retries: 5
+      start_period: 5s
+    init: true
+    read_only: true
+    tmpfs:
+      - /tmp
+    security_opt:
+      - no-new-privileges:true
+    cap_drop:
+      - ALL
+    restart: unless-stopped
+
+  gateway:
+    build:
+      context: ./backend
+    command: ["uvicorn", "app.gateway:app", "--host", "0.0.0.0", "--port", "8000"]
+    depends_on:
+      api:
+        condition: service_healthy
+    ports:
+      - "127.0.0.1:8000:8000"
+    networks:
+      - edge
+      - data
+    healthcheck:
+      test:
+        - CMD
+        - python
+        - -c
+        - >-
+          import urllib.request;
+          urllib.request.urlopen('http://127.0.0.1:8000/health/ready', timeout=2)
+      interval: 10s
+      timeout: 3s
+      retries: 5
+      start_period: 5s
+    init: true
+    read_only: true
+    tmpfs:
+      - /tmp
+    security_opt:
+      - no-new-privileges:true
+    cap_drop:
+      - ALL
+    restart: unless-stopped
+
   provider-smoke:
     build:
       context: ./backend