feat: drawer surgical delete with cascade preview (T98.4)

chat/web/drawer.py

@@ -1211,6 +1211,93 @@ async def switch_branch(
     return await drawer(chat_id, request, conn)
 
 
+@router.get(
+    "/chats/{chat_id}/drawer/turn/delete-preview/{event_id}",
+    response_class=HTMLResponse,
+)
+async def delete_preview(
+    chat_id: str,
+    event_id: int,
+    request: Request,
+    conn=Depends(get_conn),
+):
+    """Render an :class:`ImpactReport` for ``event_id`` as a small modal.
+
+    Read-only — :func:`compute_delete_impact` does not mutate the
+    database. The modal contains a confirmation form posting to
+    :func:`delete_turn` below; HTMX swaps the fragment into a modal
+    target on the chat page.
+    """
+    chat = get_chat(conn, chat_id)
+    if chat is None:
+        raise HTTPException(status_code=404, detail=f"chat not found: {chat_id}")
+
+    report = compute_delete_impact(conn, target_event_id=int(event_id))
+
+    # Build the modal HTML directly — the impact report is small and
+    # reusing the drawer template would require a fragment include just
+    # for this surface. Mirrors the rewind-preview style in
+    # :func:`chat.web.turns.rewind_preview`.
+    items_html = "".join(
+        f"<li><strong>{item.kind}</strong>: {item.description}</li>"
+        for item in report.cascading
+    )
+    notes_html = "".join(f"<li>{note}</li>" for note in report.notes)
+    body = (
+        "<div class='delete-impact-modal'>"
+        f"<h3>Delete event {report.target_event_id}?</h3>"
+        f"<p>This will discard {len(report.cascading)} events. Cascade:</p>"
+        f"<ul class='delete-impact-cascade'>{items_html or '<li>none</li>'}</ul>"
+        f"<ul class='delete-impact-notes'>{notes_html}</ul>"
+        f"<form hx-post='/chats/{chat_id}/drawer/turn/delete/{report.target_event_id}' "
+        "hx-target='#drawer' hx-swap='innerHTML'>"
+        "<button type='submit'>Confirm delete</button>"
+        "</form>"
+        "</div>"
+    )
+    return HTMLResponse(body)
+
+
+@router.post(
+    "/chats/{chat_id}/drawer/turn/delete/{event_id}",
+    response_class=HTMLResponse,
+)
+async def delete_turn(
+    chat_id: str,
+    event_id: int,
+    request: Request,
+    conn=Depends(get_conn),
+):
+    """Delete a turn (and everything after) by invoking the existing rewind path.
+
+    The :func:`chat.services.rewind.execute_rewind` API takes
+    ``after_event_id``: it removes events with id strictly greater than
+    that argument. To make ``event_id`` itself disappear we pass
+    ``after_event_id = event_id - 1`` — a thin adapter, not a
+    re-implementation of rewind.
+
+    A snapshot is taken before truncation (inside ``execute_rewind``)
+    so the user can recover via the snapshot index.
+    """
+    chat = get_chat(conn, chat_id)
+    if chat is None:
+        raise HTTPException(status_code=404, detail=f"chat not found: {chat_id}")
+
+    settings = request.app.state.settings
+    execute_rewind(
+        db_path=settings.db_path,
+        data_dir=settings.data_dir,
+        after_event_id=int(event_id) - 1,
+    )
+    # ``conn`` is now stale (the rewind opened its own connection and
+    # truncated/reprojected). Re-render the drawer through a fresh open
+    # so the partial reflects the truncated state.
+    from chat.db.connection import open_db
+
+    with open_db(settings.db_path) as fresh:
+        return await drawer(chat_id, request, fresh)
+
+
 @router.post(
     "/chats/{chat_id}/drawer/turn/hide/{event_id}",
     response_class=HTMLResponse,