feat(account): throttle confirm-code sends per recipient

Add an in-memory SendLimiter enforcing a one-per-minute cooldown and a
five-per-rolling-hour cap per recipient address, checked before provisioning or
sending in RequestCode, RequestLoginCode and RequestLinkCode. It guards against
email bombing and protects the relay quota. The limiter is injected in main
(nil in tests, so the domain suite is unaffected); ErrTooManyRequests maps to
HTTP 429.

backend/internal/account/ratelimit_test.go

@@ -0,0 +1,43 @@
+package account
+
+import (
+	"testing"
+	"time"
+)
+
+// TestSendLimiter checks the per-recipient cooldown and the rolling-hour cap, and
+// that recipients are throttled independently.
+func TestSendLimiter(t *testing.T) {
+	base := time.Now()
+	now := base
+	l := NewSendLimiter(time.Minute, 3)
+	l.now = func() time.Time { return now }
+
+	if !l.Allow("a") {
+		t.Fatal("send 1 should be allowed")
+	}
+	if l.Allow("a") {
+		t.Fatal("immediate resend must be blocked by the cooldown")
+	}
+	if !l.Allow("b") {
+		t.Fatal("a different recipient is independent")
+	}
+
+	now = base.Add(time.Minute)
+	if !l.Allow("a") {
+		t.Fatal("send 2 after the cooldown should be allowed")
+	}
+	now = base.Add(2 * time.Minute)
+	if !l.Allow("a") {
+		t.Fatal("send 3 should be allowed")
+	}
+	now = base.Add(3 * time.Minute)
+	if l.Allow("a") {
+		t.Fatal("send 4 within the hour must be blocked by the cap")
+	}
+
+	now = base.Add(time.Hour + time.Minute)
+	if !l.Allow("a") {
+		t.Fatal("after the rolling hour the cap resets")
+	}
+}