feat(kanban): add POST /runs/{run_id}/terminate endpoint

Closes the termination-control gap left by PR #28432, which shipped the
read-only sibling endpoints (/workers/active, /runs/{run_id},
/runs/{run_id}/inspect) but no way to stop a misbehaving worker from
the dashboard without dropping to the CLI.

The new endpoint resolves run_id -> task_id and delegates to the
existing kanban_db.reclaim_task() flow, so the SIGTERM->SIGKILL
escalation, run-outcome bookkeeping, and event-log append all match
POST /tasks/{task_id}/reclaim exactly. No new termination semantics
introduced.

Responses:
  200 {ok, run_id, task_id} on success
  404 unknown run_id
  409 run already ended OR task no longer reclaimable

Refs: #23762

tests/plugins/test_kanban_worker_runs.py

@@ -4,6 +4,7 @@ Covers:
   GET /workers/active
   GET /runs/{run_id}
   GET /runs/{run_id}/inspect
+  POST /runs/{run_id}/terminate
 """
 
 from __future__ import annotations
@@ -299,3 +300,141 @@ def test_inspect_run_live_pid(client, monkeypatch):
     assert body["memory_rss_bytes"] == fake_mem.rss
     assert body["num_threads"] == 4
     assert body["status"] == "sleeping"
+
+
+# ---------------------------------------------------------------------------
+# POST /runs/{run_id}/terminate
+# ---------------------------------------------------------------------------
+
+def _setup_running_task_with_run(conn, *, title, assignee, worker_pid):
+    """Create a task in 'running' state with a matching open task_runs row.
+
+    Mirrors what dispatcher_claim does: stamps tasks.status='running',
+    tasks.claim_lock, tasks.worker_pid; inserts task_runs row with the
+    same claim_lock so reclaim_task's preconditions are satisfied.
+    """
+    task_id = kb.create_task(conn, title=title, assignee=assignee)
+    lock = secrets.token_hex(8)
+    future = int(time.time()) + 3600
+    conn.execute(
+        "UPDATE tasks SET status='running', claim_lock=?, "
+        "claim_expires=?, worker_pid=? WHERE id=?",
+        (lock, future, worker_pid, task_id),
+    )
+    cur = conn.execute(
+        "INSERT INTO task_runs "
+        "(task_id, status, claim_lock, claim_expires, worker_pid, started_at) "
+        "VALUES (?, 'running', ?, ?, ?, ?)",
+        (task_id, lock, future, worker_pid, int(time.time())),
+    )
+    conn.commit()
+    return task_id, cur.lastrowid
+
+
+def test_terminate_run_404_unknown_id(client):
+    """POST to unknown run_id returns 404."""
+    r = client.post(
+        "/api/plugins/kanban/runs/777777/terminate",
+        json={"reason": "test"},
+    )
+    assert r.status_code == 404
+    assert "777777" in r.json()["detail"]
+
+
+def test_terminate_run_409_already_ended(client):
+    """POST against a run with ended_at set returns 409."""
+    conn = kb.connect()
+    try:
+        task_id = kb.create_task(conn, title="ended-terminate", assignee="ivy")
+        run_id = _insert_run(
+            conn, task_id, worker_pid=22222, ended_at=int(time.time()) - 30,
+        )
+    finally:
+        conn.close()
+
+    r = client.post(
+        f"/api/plugins/kanban/runs/{run_id}/terminate",
+        json={"reason": "too late"},
+    )
+    assert r.status_code == 409
+    assert "already ended" in r.json()["detail"]
+
+
+def test_terminate_run_ok(client, monkeypatch):
+    """Happy path: live run is terminated, signal fn invoked, reason recorded."""
+    conn = kb.connect()
+    try:
+        task_id, run_id = _setup_running_task_with_run(
+            conn, title="kill-me", assignee="jane", worker_pid=33333,
+        )
+    finally:
+        conn.close()
+
+    # Capture signal calls so we don't actually SIGTERM a random PID.
+    sent = []
+
+    def _fake_terminate(pid, prev_lock, *, signal_fn=None):
+        sent.append((pid, prev_lock))
+        return {"signal": "SIGTERM", "delivered": True}
+
+    monkeypatch.setattr(kb, "_terminate_reclaimed_worker", _fake_terminate)
+
+    r = client.post(
+        f"/api/plugins/kanban/runs/{run_id}/terminate",
+        json={"reason": "operator abort"},
+    )
+    assert r.status_code == 200, r.text
+    body = r.json()
+    assert body == {"ok": True, "run_id": run_id, "task_id": task_id}
+    assert sent == [(33333, sent[0][1])]
+    assert sent[0][1] is not None  # claim_lock was non-null
+
+    # Task is back to ready, claim cleared.
+    conn = kb.connect()
+    try:
+        row = conn.execute(
+            "SELECT status, claim_lock, worker_pid FROM tasks WHERE id=?",
+            (task_id,),
+        ).fetchone()
+    finally:
+        conn.close()
+    assert row["status"] == "ready"
+    assert row["claim_lock"] is None
+    assert row["worker_pid"] is None
+
+
+def test_terminate_run_409_task_not_reclaimable(client, monkeypatch):
+    """Open run row whose task is no longer claimable returns 409."""
+    conn = kb.connect()
+    try:
+        task_id = kb.create_task(conn, title="ghost-run", assignee="ken")
+        # Task left in default 'ready' state with no claim_lock — task_run
+        # exists but reclaim_task will refuse because status != 'running'
+        # and claim_lock is NULL.
+        run_id = _insert_run(conn, task_id, worker_pid=44444)
+    finally:
+        conn.close()
+
+    # Make sure no signal is ever sent on this code path.
+    def _boom(*a, **k):
+        raise AssertionError("_terminate_reclaimed_worker should not be called")
+
+    monkeypatch.setattr(kb, "_terminate_reclaimed_worker", _boom)
+
+    r = client.post(
+        f"/api/plugins/kanban/runs/{run_id}/terminate",
+        json={"reason": "stale"},
+    )
+    assert r.status_code == 409
+    assert "reclaimable" in r.json()["detail"]
+
+
+def test_terminate_run_accepts_empty_body(client):
+    """Empty JSON body (no reason) is still accepted; falls through to 404."""
+    r = client.post(
+        "/api/plugins/kanban/runs/666666/terminate",
+        json={},
+    )
+    # 404 because run doesn't exist — what we're asserting here is that
+    # the endpoint doesn't 422 on a missing 'reason' field.
+    assert r.status_code == 404