fix: add taskId validation and remove throwaway formatted[] array in createRoutine and updateRoutine

[Abhishek2005-ard]

Summary

Fixes two bugs in backend/controllers/routineController.js inside the
createRoutine and updateRoutine functions:

1. No taskId format validation — sending an invalid taskId caused a Mongoose
   cast error caught only by the generic 500 handler with no useful message.
2. Throwaway formatted[] array — endTime was computed into a separate array
   used only for overlap detection and then discarded. The actual items saved to DB
   never had endTime, creating inconsistent dead code.

Root Cause

Bug 1 — Missing taskId Validation

// ❌ Before — no validation on taskId
const newRoutine = new Routine({ userId, name, description, items });
// If items[n].taskId = "invalid-id", Mongoose throws a cast error → generic 500

[Abhishek2005-ard]

close #1502

[Abhishek2005-ard]

@aryandas2911
pls review this pr resolve conflicts sccessfull