feat: change default listen port from 3000 to 1852 (#756)

Updates the backend listen port, Vite dev proxy target, Docker EXPOSE,
compose port mapping, .env.example default, GitHub Actions smoke-test
default, healthcheck URLs, and every doc/example reference. Test fixtures
that include example URLs were updated for consistency, though their
assertions are port-agnostic.

The rate-limit value of 3000 in middleware/rateLimiters.ts and the
3000 entry in WEB_UI_PORTS (which detects user containers like Grafana)
are intentionally untouched.

docs/openapi.yaml

@@ -44,7 +44,7 @@ servers:
       host:
         default: localhost
       port:
-        default: "3000"
+        default: "1852"
 
 security:
   - bearerAuth: []
@@ -1679,7 +1679,7 @@ paths:
                 api_url:
                   type: string
                   description: Remote Sencho instance URL (required for remote nodes).
-                  example: https://sencho.example.com:3000
+                  example: https://sencho.example.com:1852
                 api_token:
                   type: string
                   description: API token for authenticating with the remote Sencho instance.