fix(jira): always return HTTP 200 from jira-test + strip trailing slash

- jira-test now returns {status: "ok"|"error", message: ...} with
  HTTP 200 so Cloudflare never intercepts the response
- jira_service strips trailing slash from URL before creating Jira
  client (avoids double-slash in REST paths)
- Frontend reads data.status field instead of HTTP status code

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/jira_service.py

@@ -142,8 +142,12 @@ def get_user_jira_client(user: User, db: Session):
 
     from atlassian import Jira
 
+    # Strip trailing slash — the Atlassian library appends paths like
+    # /rest/api/2/myself and a trailing slash causes double-slash URLs.
+    clean_url = jira_url.rstrip("/")
+
     return Jira(
-        url=jira_url,
+        url=clean_url,
         username=auth_email,
         password=user.jira_api_token,
         cloud=True,