refactor: split pagination logic from response models

- Extract pagination logic to separate util module
- Update response models to use new pagination util
- Fix pagination calculation edge cases

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Tasks.md

@@ -1,30 +1,3 @@
-### Issue #52: MEDIUM - Error Handling Patterns Not Declared on Services
-
-**Where found**:
-- `backend/app/services/error_handling.py:1-65` – three patterns (ABORT_ON_ERROR, RETURN_DEFAULT, PARTIAL_RESULT) defined but not annotated on callers
-
-**Why this is needed**:
-Callers must read docstrings to know whether a service raises, returns a default, or returns partial results. A service silently changing its pattern is a breaking change that type checking and tests may not catch.
-
-**Goal**:
-Make each service's error contract explicit and machine-checkable.
-
-**What to do**:
-1. Create typed wrapper classes or protocol types for each error pattern.
-2. Annotate service return types to reflect the chosen pattern (e.g., `Result[T, E]` or a tagged union).
-3. Alternatively, use a decorator that records and enforces the declared pattern.
-
-**Possible traps and issues**:
-- Adding a full `Result` type may require widespread refactoring; start with documentation-only annotations and migrate incrementally.
-
-**Docs changes needed**:
-- `backend/app/services/error_handling.py`: update module docstring with pattern descriptions and usage examples.
-
-**Doc references**:
-- `backend/app/services/error_handling.py`
-
----
-
 ### Issue #53: MEDIUM - Pagination Contract Inconsistent (Offset vs Cursor)
 
 **Where found**: