fix(web-app-nextcloud): probe the Context Agent without curl

Context Agent 2.10.0 (bumped in #1219) moved to a multi-stage image whose
runtime stage ships no curl ("no vim, no git, no curl, no pipx/poetry",
nextcloud/context_agent Dockerfile at v2.10.0). The curl healthcheck then
fails with `exec: "curl": executable file not found in $PATH`, the task
never leaves `starting`, and the swarm converge wait gives up on
nextcloud_context_agent (main run 40401200083, Nextcloud#2).

Switch to the http_status flavor, which speaks HTTP over bash's
/dev/tcp and still requires a 2xx/3xx answer from /heartbeat.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/web-app-nextcloud/meta/services.yml

@@ -155,7 +155,7 @@ context_agent:
     internal:
       http: 23000
   healthcheck:
-    flavor: curl
+    flavor: http_status
     path: heartbeat
     # nocheck: healthcheck-timing-override  a sidecar of a stack whose main service alone takes minutes; the value predates this default and has not been measured against it
     start_period: 20m