feat(assign): let leads manually assign operators + sync Jira; send platform as Jira label

Adds GET /users/operators (leads+admin) and wires a lead-only assign
control into the test detail header, calling the existing but
previously unreachable POST /tests/{id}/assign endpoint. Manual
assignment now also pushes the Jira ticket assignee immediately
instead of waiting for the operator to start execution.

Also adds test.platform as a kebab-case label on Jira ticket creation.

backend/tests/test_list_operators.py

@@ -0,0 +1,33 @@
+"""Tests for GET /users/operators — lead/admin-reachable operator picker list."""
+
+
+def test_red_lead_can_list_operators(client, red_lead_headers, red_tech_user, blue_tech_user, red_lead_user):
+    resp = client.get("/api/v1/users/operators", headers=red_lead_headers)
+    assert resp.status_code == 200, resp.text
+    usernames = {u["username"] for u in resp.json()}
+    assert "redtech" in usernames
+    assert "bluetech" in usernames
+    assert "redlead" in usernames
+
+
+def test_operators_list_excludes_viewers(client, red_lead_headers, db):
+    from app.auth import hash_password
+    from app.models.user import User
+
+    viewer = User(
+        username="viewer_ops", email="viewer_ops@test.com",
+        hashed_password=hash_password("x"), role="viewer", is_active=True,
+        must_change_password=False,
+    )
+    db.add(viewer)
+    db.commit()
+
+    resp = client.get("/api/v1/users/operators", headers=red_lead_headers)
+    assert resp.status_code == 200
+    usernames = {u["username"] for u in resp.json()}
+    assert "viewer_ops" not in usernames
+
+
+def test_red_tech_cannot_list_operators(client, red_tech_headers):
+    resp = client.get("/api/v1/users/operators", headers=red_tech_headers)
+    assert resp.status_code == 403