feat(traefik): each router problem says what it means, whether it's real, and what to do (#5712)

#5712 "Each router problem says what it means, whether it's real, and what
to do". A problem card now names its kind and reads its codes:
- Server errors: 500 means the app is failing; 502/503/504 means Traefik
  couldn't reach it or it timed out.
- Client errors: 404 scanners, a 401/403 login wall, 429 rate limiting, or
  499 clients giving up on a slow app.
- Slow: p95 against p50.
Each card says whether the problem is likely real, and ends with one next
step. A fault links to the swarm service behind the router, matched by
name, and traffic links to Settings → Thresholds. The wording lives in
plugins/traefik/meaning.py.

The plugin manager now hands out a loaded plugin's submodule
(loaded_plugin_submodule), so Traefik and host_agent reach Docker's
models the same way (#5775).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/host_agent/maintenance.py

@@ -385,18 +385,9 @@ def summary(job) -> str:
 
 
 def docker_models():
-    """The loaded Docker plugin's models module, or None when it isn't loaded.
-    Reached through the plugin manager: importing plugins.docker.models by its
-    path would define its tables a second time, and the app would refuse it."""
-    import importlib
-    from steward.core.plugin_manager import get_loaded_plugin_modules
-    loaded = get_loaded_plugin_modules().get("docker")
-    if loaded is None:
-        return None
-    try:
-        return importlib.import_module(f"{loaded.__name__}.models")
-    except ImportError:
-        return None
+    """The loaded Docker plugin's models module, or None when it isn't loaded."""
+    from steward.core.plugin_manager import loaded_plugin_submodule
+    return loaded_plugin_submodule("docker", "models")
 
 
 async def _swarm(session) -> tuple: