## Review 003: Backend Sessions and Breaks APIs

**Files reviewed:** src/backend/models.py, src/backend/api/sessions.py, src/backend/api/breaks.py, src/backend/api/__init__.py
**Verdict:** accept

### Findings

#### suggestion: Unused to_dict() methods on model classes
**Location:** src/backend/models.py:52-59 (Session), src/backend/models.py:92-101 (Break)
**Quote:**

```
    def to_dict(self) -> dict:
        return {
            "id": self.id,
            "start_time": self.start_time.isoformat() if self.start_time else None,
            ...
        }
```

**Read:** Both Session and Break models define `to_dict()` methods that convert the model to a dictionary with ISO-formatted timestamps. These methods are never called by the API layer; the API uses Pydantic response models (SessionResponse, BreakResponse) instead, which is the correct and modern pattern.
**Concern:** Dead code creates ambiguity for future readers. A developer maintaining this codebase will see both `to_dict()` methods and Pydantic response models and wonder which they should use. The presence of both suggests they serve different purposes, when actually only the Pydantic approach is in use. This violates the principle that code clarity is a property of the code, not the reader.
**Request:** Remove the `to_dict()` methods from both Session and Break models. The Pydantic response models (SessionResponse and BreakResponse) are the single, authoritative serialization path. If future layers need `to_dict()`, that decision can be made explicitly then.

### Approvals

- Database model design is clean: timezone-aware DateTime columns, JSON storage for settings_snapshot, CheckConstraint guards on duration and temporal ordering. Invariant enforcement at the DB level is correct.
- API layer correctly uses Pydantic validators to parse ISO 8601 strings and enforce end_time >= start_time before persistence. The `.replace('Z', '+00:00')` edge case handling shows attention to datetime string variations.
- Immutability is properly enforced: no PUT/PATCH endpoints exist; sessions and breaks are write-once. This matches the feature contract and prevents accidental history corruption.
- Error handling is correct: IntegrityError from CheckConstraint violations surfaces as 400 Bad Request, which is the appropriate HTTP semantics for a client-side validation failure.
- Response serialization is consistent and correct. All datetime fields round-trip through ISO format; settings_snapshot is preserved as-is; the skipped boolean is correctly converted from integer (SQLite) to bool (response).
- List endpoints are correctly paginated by temporal order (order_by start_time). This makes history queries predictable and enables pagination in future work.

### Cross-domain references

- The semantic validation gap on duration_seconds (accepting arbitrary client values without verification) is a contract decision, not a code bug. This should be explicitly documented in a Contract Note between the Tweedles before M6 frontend integration begins. See the `concern` from Tweedledum: Does the backend infer duration from (end_time - start_time), or accept it from the client? The choice should be made explicit and recorded on disk.
