fix(jira): campaign=Task, campaign tests=Sub-task, standalone tests=Task

Root cause: Jira rejects Task-under-Task nesting ("Please select valid
parent issue"). Campaign tickets and test tickets were both created as
Task, so nesting test under campaign failed for all 62 APT32 tests.

Fix:
- JIRA_ISSUE_TYPE_CAMPAIGN: "Epic" -> "Task" (was unused, now used)
- JIRA_ISSUE_TYPE_SUBTASK: "Sub-task" (new config key)
- auto_create_campaign_issue: uses JIRA_ISSUE_TYPE_CAMPAIGN (Task)
- auto_create_test_issue: uses Sub-task when parent_ticket_override is
  set (campaign context), Task otherwise (standalone)

Hierarchy: OFS-9107 -> Campaign (Task) -> Test (Sub-task)

backend/app/config.py

@@ -52,7 +52,8 @@ class Settings(BaseSettings):
     JIRA_IS_CLOUD: bool = True
     JIRA_DEFAULT_PROJECT: str = ""
     JIRA_ISSUE_TYPE_TEST: str = "Task"
-    JIRA_ISSUE_TYPE_CAMPAIGN: str = "Epic"
+    JIRA_ISSUE_TYPE_CAMPAIGN: str = "Task"
+    JIRA_ISSUE_TYPE_SUBTASK: str = "Sub-task"
 
     # ── Tempo Integration ─────────────────────────────────────────────
     TEMPO_ENABLED: bool = False