fix(auth): sync file-backed and in-memory owner caches on user rename (#3397)

The DB owner-rename loop in rename_user patched every SQL column named
owner, but three non-SQL stores were left behind:

1. session_manager.sessions -- in-memory Session objects carry s.owner
   set at server-boot time. get_sessions_for_user() does an exact
   s.owner == username check, so the renamed user chat sidebar goes empty
   until a server restart.

2. data/deep_research/*.json -- each completed research report is a
   standalone JSON file with an owner field. research_routes filters
   by d.get(owner) == user, making every report invisible to the
   renamed user.

3. data/memory.json -- a flat JSON array; each entry carries an owner
   field. memory_manager.load(owner=user) filters on it, so all memories
   vanish from the memory panel.

Fix: after the SQL loop, patch all three:
- iterate sm.sessions and update owner in-place (exposed via app.state)
- walk data/deep_research/*.json and rewrite owner with atomic_write_json
- update matching entries in memory.json with atomic_write_json

All three use the same case-insensitive lower() comparison the SQL loop
already uses. Each step is independently wrapped so a single failure
does not abort the others or the rename itself.

Fixes #3362

app.py

@@ -472,6 +472,7 @@ components = initialize_managers(BASE_DIR, rag_manager)
 session_manager   = components["session_manager"]
 from src.assistant_log import set_session_manager as _set_asst_sm
 _set_asst_sm(session_manager)
+app.state.session_manager = session_manager
 memory_manager    = components["memory_manager"]
 memory_vector     = components.get("memory_vector")
 upload_handler    = components["upload_handler"]