fix: preserve existing routine items during updates

[saurabhhhcodes]

Summary

• merge incoming routine items with already saved items during routine updates so partial saves do not wipe previous days
• reuse routine item validation for create/update paths, including empty payloads, minimum duration, and overlap checks
• validate the merged schedule before persisting to prevent silent data loss

Fixes #660

Testing

• node --check backend/controllers/routineController.js
• backend/node_modules/.bin/eslint backend/controllers/routineController.js
• git diff --check

GSSoC

Please add gssoc:approved, a difficulty label, and relevant type labels if this is accepted for GSSoC scoring.

[saurabhhhcodes]

Hi maintainers, quick GSSoC follow-up: if this PR is acceptable after review, could you please add gssoc:approved plus the suitable difficulty and type labels? I’m happy to adjust anything before merge if needed.

[saurabhhhcodes]

Rebased this PR on the latest main so the branch is current again.

Validation after rebase:

• git diff --check origin/main...HEAD
• node --check backend/controllers/routineController.js
• backend/node_modules/.bin/eslint backend/controllers/routineController.js

All passed locally. This should be ready for review/merge whenever you get a chance. If this is eligible for GSSoC scoring, please add gssoc:approved, the appropriate difficulty label, and type:bug since the fix prevents existing routine items from being accidentally dropped during updates.