fix: drawer delete-impact modal HTML escapes user-controllable fields (T110.2)

The delete-impact modal is built via raw f-string concatenation from the
ImpactReport — item.kind / item.description / report.notes ultimately
embed user-controllable content (turn prose, scene timestamps). A turn
with prose like "<script>alert(1)</script>" would reach the rendered
HTML verbatim. Currently safe (the fields embedded today are bounded
strings) but defense-in-depth — wrap with html.escape() so future
description changes can't smuggle markup through.

Test: tests/test_drawer_phase4.py::test_delete_impact_modal_escapes_user_controllable_strings.

tests/test_drawer_phase4.py

@@ -458,6 +458,42 @@ def test_t98_4_delete_invokes_rewind_and_drops_cascade(client, tmp_path):
             assert row is None, f"event {ev_id} should have been deleted"
 
 
+def test_delete_impact_modal_escapes_user_controllable_strings(client, tmp_path):
+    """T110.2: defense-in-depth — fields embedded in the modal HTML come
+    from event payloads (turn prose, scene timestamps, etc.) which are
+    ultimately user-controllable. Wrap them with ``html.escape`` so a
+    payload like ``<script>alert(1)</script>`` renders as inert text and
+    doesn't leak through into the rendered modal as actual markup.
+    """
+    db = tmp_path / "test.db"
+    _seed_chat(db)
+
+    # Seed a user_turn whose prose contains an HTML-script payload. The
+    # modal renders ``description = "turn N (you: <prose excerpt>)"`` so
+    # the prose flows verbatim into the cascade list <li>.
+    with open_db(db) as conn:
+        evil_id = append_and_apply(
+            conn,
+            kind="user_turn",
+            payload={
+                "chat_id": "chat_bot_a",
+                "prose": "<script>alert('xss')</script>",
+                "segments": [],
+            },
+        )
+
+    response = client.get(
+        f"/chats/chat_bot_a/drawer/turn/delete-preview/{evil_id}"
+    )
+    assert response.status_code == 200
+    body = response.text
+
+    # Raw <script> must NOT survive into the rendered HTML. The escaped
+    # form (&lt;script&gt;) is what we want to see instead.
+    assert "<script>alert" not in body
+    assert "&lt;script&gt;alert" in body
+
+
 def test_delete_turn_with_event_id_zero_returns_400(client, tmp_path):
     """T110.1: ``event_id <= 0`` is an obvious client error and must NOT
     silently rewind the entire log via ``after_event_id = -1``. The route