feat: provider-strict configs, slot-based templates, broadcast targets, email bots, command templates

Major architectural improvements:
- Provider-type enforcement: configs validated against provider type at assignment
- TemplateConfig migrated to slot-based pattern (TemplateSlot child table)
- Broadcast targets: TargetReceiver child table for multi-receiver dispatch
- EmailBot: first-class email sender entity with SMTP config, test connection
- CommandTemplateConfig: generic slot-based command response templates
- Provider capability registry: dynamic slot/event/command definitions per provider
- CommandTracker play/pause button matches NotificationTracker style

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

@@ -94,6 +94,40 @@ async def create_provider(
     return _provider_response(provider)
 
 
+@router.get("/capabilities")
+async def list_provider_capabilities():
+    """List capabilities for all registered provider types."""
+    from notify_bridge_core.providers.capabilities import get_all_capabilities
+    result = {}
+    for pt, caps in get_all_capabilities().items():
+        result[pt] = {
+            "provider_type": caps.provider_type,
+            "display_name": caps.display_name,
+            "notification_slots": caps.notification_slots,
+            "command_slots": caps.command_slots,
+            "events": caps.events,
+            "commands": caps.commands,
+        }
+    return result
+
+
+@router.get("/capabilities/{provider_type}")
+async def get_provider_capabilities(provider_type: str):
+    """Get capabilities for a provider type (events, slots, commands)."""
+    from notify_bridge_core.providers.capabilities import get_capabilities
+    caps = get_capabilities(provider_type)
+    if not caps:
+        raise HTTPException(status_code=404, detail=f"Unknown provider type: {provider_type}")
+    return {
+        "provider_type": caps.provider_type,
+        "display_name": caps.display_name,
+        "notification_slots": caps.notification_slots,
+        "command_slots": caps.command_slots,
+        "events": caps.events,
+        "commands": caps.commands,
+    }
+
+
 @router.get("/{provider_id}")
 async def get_provider(
     provider_id: int,