RED-77: One-pass due date retarget for roadmap tasks

[fs21co049adityawaghule]

Summary

• Implements one-pass due date retargeting for roadmap tasks
• When parent roadmap start date changes, all child tasks shift by the delta
• Uses database transaction for atomic updates
• Supports options to retarget completed/blocked tasks

Changes

• src/lib/roadmapRetarget.ts - Core retarget service with one-pass algorithm
• src/app/api/roadmaps/[id]/route.ts - Added PATCH endpoint for date updates
• tests/unit/lib/roadmapRetarget.test.ts - Unit tests for date calculations

Algorithm

1. Fetch current roadmap start date
2. Calculate delta (NEW - OLD)
3. Fetch all child tasks ( RoadmapTask)
4. Apply delta to each task's actual due date
5. Batch update in single transaction

API Usage

```bash
PATCH /api/roadmaps/:id
{
"startDate": "2024-02-15T00:00:00Z",
"retargetCompletedTasks": false,
"retargetLockedTasks": false
}
```

Edge Cases Handled

• Zero delta (no change) - returns early
• Completed tasks - optionally skipped
• Blocked/Skipped tasks - optionally skipped
• Missing roadmap - returns error

Testing

• All 450 unit tests pass
• Lint passes

Linked Issue

RED-77

[linear]

RED-77 RED-64B: One-pass due-date retarget for RED-64 child tracks

After PM confirms launch date, retarget epic and all child due dates in one pass while preserving dependency order and critical path sequencing.