docs(http-api): document SSE events across API endpoints

- Add SSE Events section to check endpoint documentation
- Add SSE Events section to update endpoint documentation
- Document scan and update event broadcasting conditions in events endpoint

Author: nicholas-fedor

docs/http-api/endpoints/check/index.md

@@ -84,3 +84,15 @@ The `/v1/check` endpoint returns a JSON array of container check results:
 |     200     | Check completed successfully                   |
 |     401     | Invalid or missing authentication token        |
 |     500     | Internal server error during request processing|
+
+## SSE Events
+
+When the [`/v1/events`](../events/index.md) SSE endpoint is also enabled, execution of the `v1/check` endpoint broadcasts the following events:
+
+- `scan_started`:  Broadcasted before the check begins
+- `scan_completed`: Broadcasted after the check finishes successfully
+- `scan_failed`: Broadcasted if the check encounters an error
+
+!!! Note
+    The `scan_completed` payload always reports `updated: 0` because no updates are applied.
+    The `failed` field counts containers whose per-container check returned an error.

docs/http-api/endpoints/events/index.md

@@ -48,13 +48,20 @@ The events endpoint uses a separate token from the main API token to limit blast
 
 - Started
 - Completed
+- Failed
+
+!!! Note
+    Scan events are broadcasted only for updates (HTTP API or scheduled) or checks (HTTP API).
 
 ### Update Events
 
 - Started
 - Completed
 - Failed
 
+!!! Note
+    Update events are broadcasted only for HTTP API or scheduled updates.
+
 ## Event Format
 
 Each event is a Server-Sent Event with an event type and JSON data payload:

docs/http-api/endpoints/update/index.md

@@ -307,3 +307,12 @@ services:
 
 !!! Warning
     Enabling the HTTP API with port mappings will automatically disable Watchtower's self-update functionality to prevent port conflicts during container recreation. See [Updating Watchtower](../../../getting-started/updating-watchtower/index.md#port_configuration_limitation) for more details.
+
+## SSE Events
+
+When the [`/v1/events`](../events/index.md) SSE endpoint is also enabled, the update process broadcasts the following events:
+
+- `scan_started`:  Broadcasted before the update scan begins
+- `scan_completed`:  Broadcasted after the update scan finishes
+- `scan_failed`:  Broadcasted if the update scan encounters an error
+- `image_cleanup`:  Broadcasted after image cleanup if enabled and images were removed