fix(history): scope topic analysis to authenticated owner only (#744)

Two changes close the cross-tenant topic leak in /api/conversations/topics.

The route at routes/history_routes.py:478 used get_current_user, which
returns None when no auth middleware has set request.state.current_user
(loopback-bypass, AUTH_ENABLED=false, or any path that short-circuits the
middleware). It then forwarded owner=None to analyze_topics.

The helper at src/topic_analyzer.py:21 used an 'if owner:' short-circuit
in its owner filter, so the None owner took the no-filter path and the
helper silently aggregated topic frequencies and per-snippet session_id,
session_name, role, and snippet text across every user's sessions.

analyze_topics now returns an empty result when owner is falsy. The
inner short-circuit is removed because the filter is now strict by
construction. The route is switched to require_user, which raises 401
when auth_manager.is_configured is True and the caller is anonymous,
matching the pattern used by calendar_routes, skills_routes, and other
authenticated routes.

The test test_history_topics_owner_scope.py was rewritten to drive the
real route through FastAPI's TestClient with a stub AuthMiddleware that
mirrors the loopback-bypass branch, and now asserts a strict 401 from
the route and an empty result from the helper. The previous version of
the test accepted either a 200-with-empty-topics or a 401; the strict
assertion means a future regression that drops the require_user wrapper
or re-adds the inner short-circuit is caught immediately.

routes/history_routes.py

@@ -477,10 +477,10 @@ def setup_history_routes(session_manager) -> APIRouter:
 
     @router.get("/api/conversations/topics")
     async def get_conversation_topics(request: Request) -> Dict[str, Any]:
-        from src.auth_helpers import get_current_user
-        user = get_current_user(request)
+        from src.auth_helpers import require_user
+        user = require_user(request)
         try:
-            return analyze_topics(session_manager, owner=user)
+            return analyze_topics(session_manager, owner=user or None)
         except Exception as e:
             raise HTTPException(500, f"Topic analysis failed: {e}")