fix: use host network for Docker build, remove obsolete version

Build stage uses network: host so pip can resolve DNS through host.
Removed version: "3.9" (obsolete warning).

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

backend/docker-compose.yml

@@ -1,5 +1,3 @@
-version: "3.9"
-
 services:
   postgres:
     image: postgres:16-alpine
@@ -20,6 +18,7 @@ services:
     build:
       context: .
       dockerfile: services/data-service/Dockerfile
+      network: host
     ports:
       - "8001:8000"
     environment:
@@ -32,6 +31,7 @@ services:
     build:
       context: .
       dockerfile: services/chart-service/Dockerfile
+      network: host
     ports:
       - "8002:8000"
     environment:
@@ -47,6 +47,7 @@ services:
     build:
       context: .
       dockerfile: services/template-service/Dockerfile
+      network: host
     ports:
       - "8003:8000"
     environment:
@@ -59,6 +60,7 @@ services:
     build:
       context: .
       dockerfile: services/export-service/Dockerfile
+      network: host
     ports:
       - "8004:8000"
     environment: