Phase 13: multi-host networks & images (0.14.0)

Networks and Images are now per-host, rendered as a section for the local host
plus one per registered agent (like the Stacks page).

- agent_app.py: new /agent/networks (list/inspect/containers/connect/disconnect/
  create/delete/prune) and /agent/images (list/updates/check), reusing
  network_service and a new image_service; DockerError mapped to HTTP status
  (forbidden -> 400 so the proxy doesn't treat it as a token failure).
- routers/agents.py: proxy routes at /api/agents/{id}/networks/* and
  /api/agents/{id}/images/*, audit-logging mutations.
- services/image_service.py: extracted the image-listing logic so the central
  router and the agent share it.
- Frontend: networksApi/imagesApi take an optional agentId; Networks/Images
  pages render NetworksSection/ImagesSection per host with a shared HostHeader.
  Remote "Prune unused" networks resolves the address-pool-exhaustion deploy
  error from the UI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/services/image_service.py

@@ -0,0 +1,47 @@
+"""Image listing — shared by the central images router and the agent."""
+from __future__ import annotations
+
+from docker_client import DockerError, get_client, safe_call
+from services import update_service
+
+COMPOSE_PROJECT_LABEL = "com.docker.compose.project"
+
+
+def list_images() -> list[dict]:
+    """Return one row per image tag, annotated with using-stacks + update status."""
+    try:
+        client = get_client()
+        images = safe_call(client.images.list)
+        containers = safe_call(client.containers.list, all=True)
+    except DockerError:
+        return []
+
+    # Map image ref -> stacks using it.
+    usage: dict[str, set[str]] = {}
+    for c in containers:
+        ref = c.attrs.get("Config", {}).get("Image")
+        stack = c.labels.get(COMPOSE_PROJECT_LABEL)
+        if ref:
+            usage.setdefault(ref, set())
+            if stack:
+                usage[ref].add(stack)
+
+    cache = update_service.get_cache()
+    result = []
+    for img in images:
+        tags = img.tags or []
+        if not tags:
+            continue
+        for tag in tags:
+            result.append(
+                {
+                    "id": img.short_id,
+                    "tag": tag,
+                    "size": img.attrs.get("Size", 0),
+                    "created": img.attrs.get("Created"),
+                    "stacks": sorted(usage.get(tag, set())),
+                    "update": cache.get(tag),
+                }
+            )
+    result.sort(key=lambda r: r["tag"])
+    return result