Add fasting feature: FastingWindow model, tests, and views

[gracechen636]

Proposed Changes

Add FastingWindow model to represent user fasting periods, including start/end times, optional goal duration, etc.

Provide convenience properties for active status and duration calculations.

Add unit tests covering model creation, behavior, and ordering.

Related Issue(s) #2134

Do users need to run some commands in their local instances due to this PR (e.g. database migration, deployment changes)?

Maybe. This PR introduces a new model, so users may need to run database migrations:

python manage.py makemigrations python manage.py migrate