BE 1.0 - Implement get_cohorts function

[LeandroHamaguchi]

Implement the get_cohorts function to fetch all the cohorts from the database table Cohorts.

Return a list of cohort objects. Use types declared in src/lib/client/supabase/types.ts for more precise return values. Example:

[
{ year: ..., term: ..., is_active: ..., created_at: ..., id: ... },
{ year: ..., term: ..., is_active: ..., created_at: ..., id: ... },
...
]

Implement tests for this function.