OAPI: include worker's "last seen" timestamp

Include the worker's "last seen" timestamp in the worker summary, details,
and SocketIO updates.

This is different from the "updated" timestamp, which indicates the last
time the worker entry in the database was updated (which could also be
in response to a status change request from the web interface).

pkg/api/flamenco-openapi.yaml

@@ -1568,6 +1568,10 @@ components:
           type: string
           format: date-time
           description: Timestamp of last update
+        "last_seen":
+          type: string
+          format: date-time
+          description: Last time this worker was seen by the Manager.
         "status": { $ref: "#/components/schemas/WorkerStatus" }
         "previous_status": { $ref: "#/components/schemas/WorkerStatus" }
         "status_change":
@@ -1626,10 +1630,10 @@ components:
         #   type: string
         #   format: uuid
         #   description: The worker's current/last-worked-on task.
-        # "last_seen":
-        #   type: string
-        #   format: date-time
-        #   description: Last time this worker was seen by the Manager.
+        "last_seen":
+          type: string
+          format: date-time
+          description: Last time this worker was seen by the Manager.
         "version":
           type: string
           description: Version of Flamenco this Worker is running