Fix maintenance schedule mutation and day rollover bugs

[vshnbot]

Summary

Fixes two bugs in maintenance schedule handling for services with PostgreSQL dependencies (Keycloak, Nextcloud):

• Mutation bug: AddTime() used a pointer receiver, mutating the original TimeOfDay. This caused the parent service to also get the offset time.
• Day rollover bug: When the 20-minute offset crossed midnight (e.g. 23:50 → 00:10), the day of week wasn't adjusted.

Replaces AddTime() with AddDuration() that returns a day offset, and moves the offset logic into SetCustomMaintenanceSchedule().

Checklist

• Update tests.
• Link this PR to related issues.
• Merge with /merge comment.

Link: vshn/appcat#590