fleet: send log counters in heartbeats and pull the text on demand

Heartbeats carry per-level counts, the most frequent log types (a stable hash of the message) and the newest id, no text. The node keeps its last 2048 lines; moshiro.logs.fetch lets the backend pull them. The log page pulls when it is open, and the backend pulls warnings and errors once a minute after a node reports an error.

backend/openapi/v1.json

@@ -1897,7 +1897,7 @@
         "tags": [
           "admin-nodes"
         ],
-        "summary": "Recent runtime logs of a node",
+        "summary": "Recent runtime logs of a node: pulled from the node when it is online, otherwise what the backend kept",
         "operationId": "node_runtime_logs",
         "parameters": [
           {
@@ -9859,4 +9859,4 @@
       "name": "agent"
     }
   ]
-}
+}