[management, client] Add management-controlled client metrics push

Allow enabling/disabling client metrics push from the dashboard via
account settings instead of requiring env vars on every client.

- Add MetricsConfig proto message to NetbirdConfig
- Add MetricsPushEnabled to account Settings (DB-persisted)
- Expose metrics_push_enabled in OpenAPI and dashboard API handler
- Populate MetricsConfig in sync and login responses
- Client dynamically starts/stops push based on management config
- NB_METRICS_PUSH_ENABLED env var overrides management when explicitly set
- Add activity events for metrics push enable/disable

shared/management/http/api/openapi.yml

@@ -367,6 +367,10 @@ components:
           description: When true, updates are installed automatically in the background. When false, updates require user interaction from the UI.
           type: boolean
           example: false
+        metrics_push_enabled:
+          description: Enables or disables client metrics push for all peers in the account
+          type: boolean
+          example: false
         embedded_idp_enabled:
           description: Indicates whether the embedded identity provider (Dex) is enabled for this account. This is a read-only field.
           type: boolean