fix: auto-detect Docker network name for tunnel and container connect

Docker Compose prefixes network names with the project directory name
(e.g. 'headquarter_backend' instead of 'backend'). The previous code
hardcoded 'backend', causing 'network not found' errors.

- apps/api/src/services/docker.py: add get_backend_network_name() that
  inspects the API container (hq-api) to find the actual network name
- apps/api/src/services/docker.py: connect_container_to_network() now
  auto-detects the network name when not explicitly provided
- apps/api/src/services/tunnel.py: import and use get_backend_network_name()
- apps/api/src/api/tool_instances.py: remove explicit 'backend' arg from
  connect_container_to_network() call

Quality gates: ruff clean

apps/api/src/api/tool_instances.py

@@ -1733,7 +1733,7 @@ async def start_instance(
     logger.debug(
         "Connecting container %s to backend network...", expected_container_name
     )
-    connected = connect_container_to_network(expected_container_name, "backend")
+    connected = connect_container_to_network(expected_container_name)
     if connected:
         logger.debug(
             "Successfully connected %s to backend network", expected_container_name