Fixes datetime comparison error when last_seen_at from database is
offset-naive (no timezone info) while 'now' is offset-aware (UTC).
This resolves the TypeError: can't subtract offset-naive and
offset-aware datetimes issue in integration tests.
- New app/services/dependency_check.py with check_milestone_deps()
- Replaces 3x duplicated JSON-parse + query + filter logic
- Supports both milestone and task dependency checking
- Returns structured DepCheckResult with ok/blockers/reason
- Refactored preflight and start endpoints to use shared helper