refactor(backend): standardize model-to-response conversion naming

Issue #8: Inconsistent naming for model-to-response conversion functions.

Changes:
- Rename build_invite_response to _to_invite_response (invites.py)
- Rename _map_counter_record to _to_counter_record_response (audit.py)
- Rename _map_sum_record to _to_sum_record_response (audit.py)

All conversion functions now follow the _to_X_response pattern,
using underscore prefix for module-private functions.

backend/routes/invites.py

@@ -31,7 +31,7 @@ admin_router = APIRouter(prefix="/api/admin", tags=["admin"])
 MAX_INVITE_COLLISION_RETRIES = 3
 
 
-def build_invite_response(invite: Invite) -> InviteResponse:
+def _to_invite_response(invite: Invite) -> InviteResponse:
     """Build an InviteResponse from an Invite with loaded relationships."""
     return InviteResponse(
         id=invite.id,
@@ -150,7 +150,7 @@ async def create_invite(
             status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
             detail="Failed to create invite",
         )
-    return build_invite_response(invite)
+    return _to_invite_response(invite)
 
 
 @admin_router.get("/invites", response_model=PaginatedInviteRecords)
@@ -197,7 +197,7 @@ async def list_all_invites(
     invites = result.scalars().all()
 
     # Build responses using preloaded relationships
-    records = [build_invite_response(invite) for invite in invites]
+    records = [_to_invite_response(invite) for invite in invites]
 
     return create_paginated_response(records, total, page, per_page)
 
@@ -230,7 +230,7 @@ async def revoke_invite(
     await db.commit()
     await db.refresh(invite)
 
-    return build_invite_response(invite)
+    return _to_invite_response(invite)
 
 
 # All routers from this module for easy registration