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/app/schemas/user.py

@@ -284,3 +284,13 @@ class UserOut(BaseModel):
         self.jira_token_set = bool(self.jira_api_token)
         self.tempo_token_set = bool(self.tempo_api_token)
         return self
+
+
+class UserOperatorOut(BaseModel):
+    """Minimal user representation for lead/admin operator-picker dropdowns."""
+
+    id: uuid.UUID
+    username: str
+    role: str
+
+    model_config = ConfigDict(from_attributes=True)