OAPI: change task log fetching from providing the log to providing metadata

The task log API endpoint was loading the entire log into RAM, then sending
it as response. This makes display in a browser also a bit harder.

The API endpoint now returns some JSON with info about the task log,
including its size and which URL can be used to download it.

Manifest task: T99730

pkg/api/flamenco-openapi.yaml

@@ -777,10 +777,10 @@ paths:
                 $ref: "#/components/schemas/Error"
 
   /api/v3/tasks/{task_id}/log:
-    summary: Fetch the task's entire log.
+    summary: Retrieve info about the task log.
     get:
-      operationId: fetchTaskLog
-      summary: Fetch the entire task log.
+      operationId: fetchTaskLogInfo
+      summary: Get the URL of the task log, and some more info.
       tags: [jobs]
       parameters:
         - name: task_id
@@ -789,11 +789,11 @@ paths:
           schema: { type: string, format: uuid }
       responses:
         "200":
-          description: The task log.
+          description: The task log info.
           content:
-            text/plain:
+            application/json:
               schema:
-                type: string
+                $ref: "#/components/schemas/TaskLogInfo"
         "204":
           description: Returned when the task has no log yet.
         default:
@@ -1542,6 +1542,20 @@ components:
         updated: { type: string, format: date-time }
       required: [id, name, status, priority, task_type, updated]
 
+    TaskLogInfo:
+      type: object
+      description: Info about the log of a single task.
+      properties:
+        "task_id": { type: string, format: uuid }
+        "job_id": { type: string, format: uuid }
+        "url":
+          description: The URL at which the task log itself can be downloaded.
+          type: string
+        "size":
+          description: The size of the task log, in bytes.
+          type: integer
+      required: [task_id, job_id, url, size]
+
     JobLastRenderedImageInfo:
       description: >
         Enough information for a client to piece together different strings to