feat(backend): implement service monitor proto & connect it to http /agents

backend/docs/swagger.yaml

@@ -306,14 +306,24 @@ definitions:
   internal_handlers.AgentInfo:
     properties:
       connected_at:
+        description: Time when agent connected (RFC3339-like)
+        example: "2026-04-04 10:30:00"
         type: string
       label:
+        description: Human-readable label
+        example: web-server-1
         type: string
       services:
+        description: 'List of services with status (format: "name:status")'
+        example:
+        - nginx:running
+        - redis:up
         items:
           type: string
         type: array
       token:
+        description: Unique agent identifier
+        example: agent-001
         type: string
     type: object
   internal_handlers.InsertLogRequest:
@@ -397,6 +407,8 @@ info:
 paths:
   /agents:
     get:
+      consumes:
+      - application/json
       description: Returns a list of all agents currently connected via Collector
         (log streaming)
       produces: