refactor: introduce clock to make tests reproducible

internal/clock/clock.go

@@ -0,0 +1,28 @@
+package clock
+
+import "time"
+
+// Clock is the time source injected into services.
+// Production code uses Real(); tests use Fixed().
+type Clock interface {
+	Now() time.Time
+}
+
+// Real returns a Clock backed by time.Now().
+func Real() Clock { return realClock{} }
+
+type realClock struct{}
+
+func (realClock) Now() time.Time { return time.Now() }
+
+// FixedClock always returns T. Advance shifts T forward.
+// Intended for tests only.
+type FixedClock struct{ T time.Time }
+
+// Fixed returns a *FixedClock set to t.
+func Fixed(t time.Time) *FixedClock { return &FixedClock{T: t} }
+
+func (f *FixedClock) Now() time.Time { return f.T }
+
+// Advance shifts the clock forward by d.
+func (f *FixedClock) Advance(d time.Duration) { f.T = f.T.Add(d) }