refactor(plugins): the display publishes plugin runtime state; retire plugin_state.json (#690)

Stage 2 of the web plugin catalog, after #688.

- The display publishes a plugin runtime snapshot (plugin_runtime.py) to
  the shared cache: per plugin loaded, lifecycle state, a short redacted
  error summary, the version it loaded and when, plus published_at /
  stale_after / running. Written on change (throttled to 10 s; the
  RUNNING/ENABLED flip of an ordinary update is not a change) and once a
  minute otherwise; cleanup() publishes running: false.
- The web reads it back and restores loaded / state / error_info in
  /api/v3/plugins/installed (plus loaded_version, loaded_at and
  data.runtime). Only a live snapshot counts; stale, stopped or missing
  answers null and says which.
- data/plugin_state.json is retired: every reader and writer moved to
  config + disk (desired) or the snapshot (observed). Nothing in it was
  non-derivable, so nothing is migrated and an existing file is left
  unread. The web-side PluginStateManager (state_manager.py) is removed;
  the display's plugin_state.PluginStateManager is the only state machine.
- StateReconciliation compares config + disk with the snapshot, reporting
  enabled-but-not-loaded and older-version-loaded as no_action findings.
- Backups list installed manifests with enabled from config.json.

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

test/test_reconciliation_phantom_plugins.py

@@ -54,9 +54,7 @@ class _ConfigManager:
 
 def _reconciler(config, plugins_dir, secrets_path=None):
     return StateReconciliation(
-        state_manager=Mock(),
         config_manager=_ConfigManager(config, secrets_path),
-        plugin_manager=Mock(),
         plugins_dir=plugins_dir,
     )
 
@@ -92,9 +90,8 @@ class TestSecretsKeysAreNotPlugins:
         assert "odds-ticker" in r._get_config_state()
 
     def test_config_manager_without_secrets_path_is_survivable(self, tmp_path):
-        r = StateReconciliation(state_manager=Mock(),
-                                config_manager=Mock(spec=["load_config", "save_config"]),
-                                plugin_manager=Mock(), plugins_dir=tmp_path)
+        r = StateReconciliation(config_manager=Mock(spec=["load_config", "save_config"]),
+                                plugins_dir=tmp_path)
         r.config_manager.load_config.return_value = {"odds-ticker": {"enabled": True}}
         assert "odds-ticker" in r._get_config_state()