## Implementation 002: Fix unresolved enum imports in history.py

**Side:** backend
**Ticket:** none — bug fix on shipped code
**Contract:** session-history-v1 (/sessions/history endpoint returns HistorySessionResponse list with completed sessions, break metadata, and durations)
**Ready for review:** yes

**Approach:**

Removed unnecessary SessionState and BreakState enum imports from history.py. Changed state comparisons from `Session.state == SessionState.COMPLETED` to `Session.state == "completed"` and `break_obj.state == BreakState.SKIPPED` to `break_obj.state == "skipped"`. This aligns with the pattern used throughout sessions.py, breaks.py, and statistics.py, where state values are stored as strings in the database and compared as strings in queries.

**Invariants Enforced:**
- Session must be in 'completed' state to appear in history — enforced by query filter
- Break duration and skip status accurately reflect the break record associated with the session

**Schema Changes:**

none — no migrations required

**Failure Modes Handled:**
- No break found for a session: returns break_duration_seconds=0 and break_skipped=false
- Null timestamps (defensive): defaults to datetime.now(timezone.utc) if either start_time or completed_at is missing

**Files:**
- src/backend/api/history.py: Removed enum imports, changed state comparisons to string literals
