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/services/docker.py

@@ -230,18 +230,54 @@ def get_container_name(instance_name: str) -> str | None:
     return None
 
 
+def get_backend_network_name() -> str:
+    """Auto-detect the actual Docker network name for the backend network.
+
+    Docker Compose prefixes network names with the project directory name
+    (e.g. 'headquarter_backend' instead of 'backend'). We inspect the API
+    container itself to find the real network name it's connected to.
+
+    Returns:
+        The actual Docker network name, or 'backend' as fallback.
+    """
+    # Try to find the API container by its known name
+    api_container = "hq-api"
+    result = subprocess.run(
+        [
+            "docker",
+            "inspect",
+            "-f",
+            "{{range $k, $v := .NetworkSettings.Networks}}{{$k}} {{end}}",
+            api_container,
+        ],
+        capture_output=True,
+        text=True,
+    )
+    if result.returncode == 0 and result.stdout.strip():
+        networks = result.stdout.strip().split()
+        for net in networks:
+            if "backend" in net.lower():
+                return net
+        # API container is on some network — return the first one
+        return networks[0]
+    return "backend"
+
+
 def connect_container_to_network(
-    container_name: str, network_name: str = "backend"
+    container_name: str, network_name: str | None = None
 ) -> bool:
     """Connect a Docker container to an existing network.
 
     Args:
         container_name: Name or ID of the container
-        network_name: Name of the Docker network (default: backend)
+        network_name: Name of the Docker network. If None, auto-detects
+            from the API container's own network membership.
 
     Returns:
         True if successful, False otherwise
     """
+    if network_name is None:
+        network_name = get_backend_network_name()
     result = subprocess.run(
         ["docker", "network", "connect", network_name, container_name],
         capture_output=True,

apps/api/src/services/tunnel.py

@@ -12,10 +12,11 @@ import re
 import subprocess
 from typing import Any
 
+from src.services.docker import get_backend_network_name
+
 logger = logging.getLogger(__name__)
 
 TUNNEL_IMAGE = "cloudflare/cloudflared:latest"
-TUNNEL_NETWORK = "backend"
 
 
 def _tunnel_container_name(instance_name: str) -> str:
@@ -105,7 +106,7 @@ def start_tunnel(
         "run",
         "-d",
         "--network",
-        TUNNEL_NETWORK,
+        get_backend_network_name(),
         "--name",
         tunnel_name,
         TUNNEL_IMAGE,