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/REST_API_REFERENCE.md

@@ -40,8 +40,9 @@ the entry below says so.
 
 > The API blueprint is the `api_v3` package in
 > `web_interface/blueprints/api_v3/` (one module per area: `config.py`,
-> `display.py`, `plugins.py`, `system.py`, `backup.py`, `fonts.py`,
-> `misc.py`, `wifi.py`, `starlark.py`). `web_interface/app.py` registers it
+> `display.py`, `system.py`, `backup.py`, `fonts.py`, `misc.py`, `wifi.py`,
+> `starlark.py`, and `plugins.py` plus the `plugin_*.py` modules for the
+> plugin routes). `web_interface/app.py` registers it
 > at `/api/v3` (`app.register_blueprint(api_v3, url_prefix='/api/v3')`).
 > The three SSE endpoints (`/api/v3/stream/*`) are defined directly on the
 > Flask app in `app.py` (`stream_stats`, `stream_display`, `stream_logs`).