Cleanup: Move defaultdict imports to top of file

[samkeen]

Issue

The from collections import defaultdict import was added inside function bodies in multiple places:

• api/src/routes/projects.py - lines 368, 537, 650, 1021
• api/src/routes/activity.py - line 143

Fix

Move these imports to the top of each file with the other imports.

Note

In get_session() (projects.py ~line 650), the import is unused - sub_agent_ids is built with a regular list, not defaultdict. This import can be removed entirely from that function.