calendar subscription: increase calculated shifts from 1 month to 1 year

[m17ch]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
This PR increases the calendar subscription window of calculated shifts from 1 month to 1 year.

Which issue(s) this PR fixes:
Fixes #1459

Describe any introduced user-facing changes:
This change will not require any action from any users that have existing subscriptions created. The next time a calendar agent fetches from subscription endpoint, data for the full year will be pulled.

Notes:
An analysis of impact using a worst case scenario for number of shifts (hourly rotation with 2 users, on call 24x7) resulted in a payload of 1.2 MB for 1 year (from 102 KB for 1 month). No noticeable detrimental effect on query execution times or request duration on calendar subscription endpoint. Application logs were reviewed and real-world payload sizes were substantially smaller than test case above, so the increase in window duration should not be impactful.

[Forfold]

lgtm, and thanks for taking the time to thoroughly review any potential impacts!