feat(tests): let managers escalate and directly resolve disputed tests

Request Discussion only ever nudged the peer lead, and the one-time
manager notification when a dispute starts had no real follow-up
action attached to it — a manager could be told a dispute existed but
had no way to actually do anything about it. Adds:

- POST /tests/{id}/escalate-to-manager — either lead can explicitly
  ask managers to step in, distinct from the passive initial notice.
- POST /tests/{id}/manager-resolve-dispute — a manager decides the
  final outcome (validated/rejected) directly, bypassing both leads'
  votes entirely, reusing the same dual-validation event dispatch so
  Jira/notifications behave like any other resolution.

Both leads are notified of the manager's final call, win or lose.

backend/app/schemas/test.py

@@ -185,6 +185,13 @@ class TestResolveDispute(BaseModel):
     notes: str | None = None
 
 
+class TestManagerResolveDispute(BaseModel):
+    """Payload sent by a manager making the final call on a disputed test."""
+
+    outcome: str  # "validated" | "rejected"
+    notes: str | None = None
+
+
 # ── Red Lead review gate (pre-Blue-Team) ────────────────────────────