✨ feat(bdd): pkg/bdd/mailpit/ HTTP client + integration tests (ADR-0030 Phase A.2)

Adds the Mailpit HTTP API client used by BDD scenarios to assert on
emails sent during a test. Implements the per-recipient query/await/
purge pattern from ADR-0030.

Build-tag-gated integration tests run against the live Mailpit at
localhost:8025 (started via docker compose up -d mailpit).

Three operations on the client:
- MessagesTo(ctx, to)        — list message IDs for a recipient
- Get(ctx, id)               — fetch full message content (text, html,
                               headers, subject, etc.)
- AwaitMessageTo(ctx, to,    — poll until a message arrives or timeout
   timeout)                    (50ms polls, fail-fast on ctx cancel)
- PurgeMessagesTo(ctx, to)   — delete all messages for a recipient

Tests in client_integration_test.go (build tag `integration`):
- RoundTrip: SMTP submit → list → get → assert subject/text — proves
  the BDD-helper contract end-to-end via real SMTP
- AwaitTimeoutWhenNoMessage: bounded wait when no email arrives
- PurgeIsolation: per-recipient delete does NOT affect other recipients

Mailpit API quirk discovered + documented (Q-NNN candidate):
- /api/v1/messages?query=... is for PAGINATION, not filtering — the
  `query` param there is silently ignored for filtering
- /api/v1/search?query=to:<addr> is the correct endpoint for filtering
  AND the matching DELETE
- ADR-0030 + EMAIL.md updated with the correct endpoint

Run integration tests:
  docker compose up -d mailpit
  go test -tags integration -race ./pkg/bdd/mailpit/...

Out of scope for this PR (Phase A.3+):
- pkg/bdd/steps/email_steps.go BDD step definitions
- magic_link_tokens table + repository
- magic-link/request and magic-link/consume HTTP handlers
- BDD scenarios for the magic-link flow

pkg/bdd/mailpit/client_integration_test.go

@@ -0,0 +1,133 @@
+//go:build integration
+
+// Integration tests for the Mailpit client. Run with:
+//
+//	go test -tags integration ./pkg/bdd/mailpit/...
+//
+// Requires a running Mailpit reachable at http://localhost:8025
+// (the docker-compose service from ADR-0029).
+package mailpit
+
+import (
+	"context"
+	"crypto/rand"
+	"encoding/hex"
+	"net/smtp"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// uniqueRecipient returns an address unique to this test run, using the
+// per-scenario-recipient pattern from ADR-0030. Two parallel test runs
+// generate different suffixes so they never see each other's messages.
+func uniqueRecipient(t *testing.T) string {
+	t.Helper()
+	var raw [4]byte
+	_, err := rand.Read(raw[:])
+	require.NoError(t, err)
+	return "integ-" + t.Name() + "-" + hex.EncodeToString(raw[:]) + "@bdd.local"
+}
+
+// sendViaSMTP submits a small email through Mailpit's SMTP port.
+// Real-wire-format path : same as the application code will use.
+func sendViaSMTP(t *testing.T, to, subject, body string) {
+	t.Helper()
+	from := "integ-test@bdd.local"
+	msg := []byte(
+		"From: " + from + "\r\n" +
+			"To: " + to + "\r\n" +
+			"Subject: " + subject + "\r\n" +
+			"\r\n" +
+			body + "\r\n",
+	)
+	err := smtp.SendMail("localhost:1025", nil, from, []string{to}, msg)
+	require.NoError(t, err, "SMTP send to local Mailpit")
+}
+
+// TestIntegration_RoundTrip validates the full path : SMTP submit →
+// Mailpit captures → client lists → client gets full body. This is
+// the smoke test for the BDD-helper contract.
+func TestIntegration_RoundTrip(t *testing.T) {
+	c := NewClient()
+	to := uniqueRecipient(t)
+
+	// Defensive cleanup before the test (in case the recipient was reused)
+	require.NoError(t, c.PurgeMessagesTo(context.Background(), to))
+
+	subject := "Integration roundtrip"
+	body := "Token: integ-token-" + strings.ReplaceAll(to, "@", "-at-")
+
+	sendViaSMTP(t, to, subject, body)
+
+	msg, err := c.AwaitMessageTo(context.Background(), to, 3*time.Second)
+	require.NoError(t, err)
+	require.NotNil(t, msg)
+
+	assert.Equal(t, subject, msg.Subject)
+	assert.Contains(t, msg.Text, "Token: integ-token-")
+	if assert.Len(t, msg.To, 1) {
+		assert.Equal(t, to, msg.To[0].Address)
+	}
+
+	// Cleanup so subsequent runs of this same test name don't accumulate
+	require.NoError(t, c.PurgeMessagesTo(context.Background(), to))
+}
+
+// TestIntegration_AwaitTimeoutWhenNoMessage confirms AwaitMessageTo
+// returns an error within the timeout when no message arrives.
+func TestIntegration_AwaitTimeoutWhenNoMessage(t *testing.T) {
+	c := NewClient()
+	to := uniqueRecipient(t) // never sent to → must time out
+
+	start := time.Now()
+	_, err := c.AwaitMessageTo(context.Background(), to, 200*time.Millisecond)
+	elapsed := time.Since(start)
+
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "no message")
+	assert.GreaterOrEqual(t, elapsed, 150*time.Millisecond, "should poll until close to timeout")
+	assert.Less(t, elapsed, 1*time.Second, "should not exceed timeout substantially")
+}
+
+// TestIntegration_PurgeIsolation proves the per-recipient query/delete
+// model from ADR-0030 : two unique recipients can have their own
+// messages without one's purge affecting the other.
+func TestIntegration_PurgeIsolation(t *testing.T) {
+	c := NewClient()
+	// Build two distinct, well-formed addresses (separate local-parts,
+	// same domain). Avoid mutating uniqueRecipient's output post-@.
+	var rawA, rawB [4]byte
+	_, _ = rand.Read(rawA[:])
+	_, _ = rand.Read(rawB[:])
+	toA := "iso-a-" + hex.EncodeToString(rawA[:]) + "@bdd.local"
+	toB := "iso-b-" + hex.EncodeToString(rawB[:]) + "@bdd.local"
+
+	sendViaSMTP(t, toA, "for A", "body A")
+	sendViaSMTP(t, toB, "for B", "body B")
+
+	// Both messages should exist
+	idsA, err := c.MessagesTo(context.Background(), toA)
+	require.NoError(t, err)
+	assert.GreaterOrEqual(t, len(idsA), 1, "A should have its message")
+	idsB, err := c.MessagesTo(context.Background(), toB)
+	require.NoError(t, err)
+	assert.GreaterOrEqual(t, len(idsB), 1, "B should have its message")
+
+	// Purge A only
+	require.NoError(t, c.PurgeMessagesTo(context.Background(), toA))
+
+	// A is empty, B is untouched
+	idsA, err = c.MessagesTo(context.Background(), toA)
+	require.NoError(t, err)
+	assert.Empty(t, idsA, "A should be empty after purge")
+	idsB, err = c.MessagesTo(context.Background(), toB)
+	require.NoError(t, err)
+	assert.GreaterOrEqual(t, len(idsB), 1, "B should still have its message")
+
+	// Cleanup B
+	require.NoError(t, c.PurgeMessagesTo(context.Background(), toB))
+}