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/ARCHITECTURE.md

@@ -158,8 +158,13 @@ everything else through `_reinstall_with_rollback()`.
 - **API.** [`blueprints/api_v3/`](../web_interface/blueprints/api_v3/) is one
   blueprint at `/api/v3`, split by area: `backup.py`, `config.py`,
   `display.py`, `fonts.py`, `misc.py` (health, logs, errors, cache, sync),
-  `plugins.py`, `starlark.py`, `system.py` (service actions, updates, git),
-  `wifi.py`. `__init__.py` defines the blueprint and shared helpers and
+  `starlark.py`, `system.py` (service actions, updates, git), `wifi.py`, and
+  the plugin routes: `plugins.py` (installed list, enable/disable, plugin
+  actions), `plugin_store.py` (install, update, uninstall, store),
+  `plugin_config.py` (config, schema, reset), `plugin_assets.py` (uploads,
+  plugin static files), `plugin_health.py` (health, metrics, limits),
+  `plugin_operations.py` (operation history, state reconciliation) and
+  `plugin_calendar.py`. `__init__.py` defines the blueprint and shared helpers and
   imports the modules so their routes register. Endpoints are listed in
   [REST_API_REFERENCE.md](REST_API_REFERENCE.md).
 - **Front end.** HTMX loads each tab's partial on first open