refactor(web): split api_v3/plugins.py by area

web_interface/blueprints/api_v3/plugins.py (3,285 lines) becomes:
- plugins.py: installed list, enable/disable, plugin actions
- plugin_store.py: install, update, uninstall, store, saved repositories
- plugin_config.py: config get/save, schema, reset
- plugin_assets.py: asset uploads and plugin static files
- plugin_health.py: health, metrics, limits
- plugin_operations.py: operation history, state reconciliation
- plugin_calendar.py: calendar credentials and auth

Pure move: all 44 functions and 38 route decorators are byte-identical
(checked with ast), URLs and endpoint names are unchanged (url-map test).
Each module imports only what it uses. Tests and config.py that reached
into plugins.py for moved names now import from the new module.

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

docs/PLUGIN_CONFIGURATION_TABS.md

@@ -203,7 +203,7 @@ Forms are rendered on the server, not generated in the browser:
    from the schema (widgets named by `x-widget` are rendered by the scripts in
    `web_interface/static/v3/js/widgets/`)
 4. **Save Configuration** posts the form to `/api/v3/plugins/config`
-   (`web_interface/blueprints/api_v3/plugins.py`), which validates it against
+   (`web_interface/blueprints/api_v3/plugin_config.py`), which validates it against
    the schema, writes `config.json` (secret fields go to
    `config_secrets.json`) and shows a notification