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/audit.py

@@ -61,7 +61,9 @@ async def paginate_with_user_email(
     return records, total, calculate_total_pages(total, per_page)
 
 
-def _map_counter_record(record: CounterRecord, email: str) -> CounterRecordResponse:
+def _to_counter_record_response(
+    record: CounterRecord, email: str
+) -> CounterRecordResponse:
     return CounterRecordResponse(
         id=record.id,
         user_email=email,
@@ -71,7 +73,7 @@ def _map_counter_record(record: CounterRecord, email: str) -> CounterRecordRespo
     )
 
 
-def _map_sum_record(record: SumRecord, email: str) -> SumRecordResponse:
+def _to_sum_record_response(record: SumRecord, email: str) -> SumRecordResponse:
     return SumRecordResponse(
         id=record.id,
         user_email=email,
@@ -91,7 +93,7 @@ async def get_counter_records(
 ) -> PaginatedCounterRecords:
     """Get paginated counter action records."""
     records, total, _ = await paginate_with_user_email(
-        db, CounterRecord, page, per_page, _map_counter_record
+        db, CounterRecord, page, per_page, _to_counter_record_response
     )
     return create_paginated_response(records, total, page, per_page)
 
@@ -105,7 +107,7 @@ async def get_sum_records(
 ) -> PaginatedSumRecords:
     """Get paginated sum action records."""
     records, total, _ = await paginate_with_user_email(
-        db, SumRecord, page, per_page, _map_sum_record
+        db, SumRecord, page, per_page, _to_sum_record_response
     )
     return create_paginated_response(records, total, page, per_page)