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>

docs/plugins/traefik.md

@@ -84,6 +84,11 @@ The full Traefik page at `/plugins/traefik/` reads top to bottom, for the time r
 1. **The verdict.** It names the routers that need a look, says why, and says when the problem started ("api, 3.0% of responses are 5xx, started 2h ago"). When a router has been a problem for the whole range, it says so instead. Otherwise it reads "All N routers healthy".
 2. **The request chart.** Requests per second across all routers and the share answered 5xx, the same chart as the dashboard widget.
 3. **A card for each router that needs a look:** why, since when, the 4xx and 5xx status codes it answered in the last 15 minutes, most first ("404 ×1,812, 502 ×40"), and its request rate, bandwidth, p95/p99 latency and 4xx/5xx shares, each with its history. The code is what separates scanners probing for pages (404), a login wall (401/403) and an app that isn't answering (502/503/504). Steward records codes from the version that added them on; until then a card says they aren't recorded yet.
+   Each card then says what its problem usually means and what to do (`plugins/traefik/meaning.py`, #5712):
+   - **Server errors (5xx)** are almost always real. Mostly 500: the app is failing requests. Mostly 502/503/504: Traefik couldn't reach the app, or it timed out.
+   - **Client errors (4xx)** are usually traffic rather than a fault. Mostly 404: scanners and bots. 401/403: a login wall. 429: rate limiting. Mostly 499 is the exception: clients giving up on a slow app.
+   - **Slow responses** compare p95 with p50. When p95 is 4× p50 or more, a few requests are slow (one endpoint or a heavy page). When p50 is close behind, the whole app is slow.
+   - **Next:** a fault links to the swarm service behind the router (its tasks and logs) when Steward can match the name ("codex@swarm" → `codex_app`, "radarr@swarm" → `arr_radarr`), otherwise to Docker → Swarm. Traffic links to Settings → Thresholds.
 4. **Traefik's own figures:** when its config last reloaded, open connections, memory.
 5. **The healthy routers,** folded into one table, then the routers Traefik no longer reports, then TLS certificates by soonest expiry, then Traffic Origin when the access log is on.