Canonical time source for Accrue.

All code inside Accrue that needs "now" calls Accrue.Clock.utc_now/0 instead of DateTime.utc_now/0. In the test environment (:accrue, :env set to :test) this delegates to Accrue.Processor.Fake.now/0, which is backed by the Fake processor's in-memory test clock. In any other environment it delegates to the BEAM's DateTime.utc_now/0.

This indirection is the mechanism that makes time-sensitive billing logic testable without sleeping or monkeypatching. Trial expiry, dunning grace windows, and expiring-card notices can all be exercised deterministically by advancing Accrue.Test.Clock in your tests rather than waiting for wall-clock time to pass.

Why application env, not compile env

The :env flag is read at runtime via Application.get_env/3 so host apps can flip it in tests without recompiling Accrue. Production never reaches the Fake branch because the default is :prod and host apps never set :accrue, :env to anything but :test.