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>

web_interface/blueprints/api_v3/config.py

@@ -1038,7 +1038,7 @@ def save_main_config():
                     )
                 schema = schema_mgr.load_schema(plugin_id, use_cache=False)
 
-                from web_interface.blueprints.api_v3.plugins import (
+                from web_interface.blueprints.api_v3.plugin_config import (
                     _merge_onto_stored_plugin_config, _prepare_plugin_config_for_save,
                 )
                 plugin_config = _merge_onto_stored_plugin_config(