feat(server): add /status command handler for webhook providers

The generic-webhook provider has no upstream API, so /status reports
DB-derived stats: active/total trackers, provider name, and last event
timestamp (formatted via the shared get_last_event_str helper).

Includes pytest coverage for handler registration, populated stats with
a recent event, the empty-state dash sentinel, and unknown-command
fall-through. Template variable docs in command_template_configs.py
extended with the new trackers_active/trackers_total keys.

packages/server/src/notify_bridge_server/api/command_template_configs.py

@@ -403,7 +403,13 @@ async def get_command_variables(
     webhook = {
         "status": {
             "description": "/status webhook provider summary",
-            "variables": {**common_vars, "provider_name": "Webhook provider name", "last_event": "Last event timestamp"},
+            "variables": {
+                **common_vars,
+                "trackers_active": "Number of enabled trackers attached to the webhook provider",
+                "trackers_total": "Total number of trackers attached to the webhook provider",
+                "provider_name": "Webhook provider name",
+                "last_event": "Last event timestamp ('YYYY-MM-DD HH:MM' or '-')",
+            },
         },
     }