feat(ui): plugin get_nav() hook → sidebar Infrastructure links (slice 2)

Plugin UIs had no nav home (only dashboard widgets / typed URLs). Add an
optional get_nav() plugin export and surface it in the sidebar.

- plugin_manager: _PLUGIN_NAV registry + get_plugin_nav() getter + a tolerant
  _collect_plugin_nav() (missing hook = fine; raising/malformed = logged &
  skipped; idempotent per plugin so hot-reload re-runs cleanly). Collected in
  both the startup load path and the hot-reload path.
- app.py: inject plugin_nav into the template context, filtered to enabled
  plugins so a hot-disabled plugin's link can't linger before restart.
- base.html: render plugin links under the Infrastructure group, with the same
  request.path active-state treatment as core links.
- docker/snmp/unifi/traefik: each exports get_nav() → its /plugins/<name>/ view.
- Tests: collector behavior (collect, missing hook, reload-replace, malformed
  item, raising hook, sorted output).

With docker enabled, "Docker" now appears under Infrastructure → its fleet view.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

plugins/snmp/__init__.py

@@ -21,3 +21,8 @@ def get_scheduled_tasks() -> list:
 def get_blueprint():
     from .routes import snmp_bp
     return snmp_bp
+
+
+def get_nav() -> list[dict]:
+    # Surfaces in the sidebar's Infrastructure group (SNMP device readings).
+    return [{"label": "SNMP", "href": "/plugins/snmp/"}]