Generalize post reply notifications (#6)

## Summary
- Notify the direct parent post author for replies throughout nested conversations
- Skip root creation, self-replies, edits, disabled mail, and recipients without email
- Link directly to each reply and use per-reply Resend idempotency

Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/web/posts_test.go

@@ -7,7 +7,9 @@ import (
 	"net/url"
 	"strings"
 	"testing"
+	"time"
 
+	"plumber/internal/mail"
 	"plumber/internal/pacific"
 	"plumber/internal/store"
 )
@@ -258,6 +260,127 @@ func TestEditPostRoutePermissions(t *testing.T) {
 	}
 }
 
+func TestPostReplyNotifications(t *testing.T) {
+	t.Parallel()
+
+	recording := &mail.Recording{}
+	srv, mem := newTestServer(t, Config{Mail: recording})
+	handler := srv.Handler()
+	homeowner := seedUser(t, mem, uniq("homeowner"), "hunter22", store.RoleUser)
+	admin := seedUser(t, mem, uniq("admin"), "hunter22", store.RoleAdmin)
+	homeownerCookies := loginUser(t, handler, homeowner.Username, "hunter22")
+	adminCookies := loginUser(t, handler, admin.Username, "hunter22")
+	homeownerCSRF := csrfForCookies(t, handler, homeownerCookies)
+	adminCSRF := csrfForCookies(t, handler, adminCookies)
+
+	rec := postForm(handler, "/posts", url.Values{
+		"_csrf": {homeownerCSRF},
+		"title": {"Leaky sink"},
+		"body":  {"Water under the cabinet."},
+	}, homeownerCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("root create status = %d: %s", rec.Code, rec.Body.String())
+	}
+	if recording.Len() != 0 {
+		t.Fatalf("root create sent %d notifications", recording.Len())
+	}
+	roots, err := mem.ListRootPosts(context.Background(), pacific.Today(), homeowner.ID)
+	if err != nil || len(roots) != 1 {
+		t.Fatalf("created roots = %+v, %v", roots, err)
+	}
+	root := roots[0]
+
+	rec = postForm(handler, "/posts", url.Values{
+		"_csrf":     {adminCSRF},
+		"parent_id": {root.ID},
+		"body":      {"Replace the cartridge."},
+	}, adminCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("admin reply status = %d: %s", rec.Code, rec.Body.String())
+	}
+	thread, err := mem.GetPostThread(context.Background(), root.ID)
+	if err != nil || len(thread.Replies) != 1 {
+		t.Fatalf("admin reply thread = %+v, %v", thread, err)
+	}
+	adminReply := thread.Replies[0]
+	msgs := waitForMail(t, recording, 1)
+	if msg := msgs[0]; msg.ToEmail != homeowner.Email ||
+		msg.RootID != root.ID ||
+		msg.RootTitle != root.Title ||
+		msg.ReplyID != adminReply.ID ||
+		msg.ReplyBody != adminReply.Body ||
+		msg.ReplyAuthorName != admin.Name {
+		t.Fatalf("admin reply notification = %+v", msg)
+	}
+
+	rec = postForm(handler, "/posts", url.Values{
+		"_csrf":     {homeownerCSRF},
+		"parent_id": {adminReply.ID},
+		"body":      {"That fixed the drip."},
+	}, homeownerCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("homeowner reply status = %d: %s", rec.Code, rec.Body.String())
+	}
+	thread, err = mem.GetPostThread(context.Background(), root.ID)
+	if err != nil || len(thread.Replies[0].Replies) != 1 {
+		t.Fatalf("homeowner nested reply thread = %+v, %v", thread, err)
+	}
+	homeownerReply := thread.Replies[0].Replies[0]
+	msgs = waitForMail(t, recording, 2)
+	if msg := msgs[1]; msg.ToEmail != admin.Email ||
+		msg.RootID != root.ID ||
+		msg.ReplyID != homeownerReply.ID ||
+		msg.ReplyAuthorName != homeowner.Name {
+		t.Fatalf("homeowner reply notification = %+v", msg)
+	}
+
+	rec = postForm(handler, "/posts", url.Values{
+		"_csrf":     {homeownerCSRF},
+		"parent_id": {root.ID},
+		"body":      {"A note to myself."},
+	}, homeownerCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("self reply status = %d: %s", rec.Code, rec.Body.String())
+	}
+	rec = postForm(handler, "/posts/"+adminReply.ID+"/edit", url.Values{
+		"_csrf": {adminCSRF},
+		"body":  {"Replace the ceramic cartridge."},
+	}, adminCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("edit status = %d: %s", rec.Code, rec.Body.String())
+	}
+
+	noEmail := &store.User{
+		Username:     uniq("no-email"),
+		PasswordHash: homeowner.PasswordHash,
+		Role:         store.RoleUser,
+	}
+	if err := mem.CreateUser(context.Background(), noEmail); err != nil {
+		t.Fatal(err)
+	}
+	noEmailRoot := &store.Post{
+		AuthorID: noEmail.ID,
+		Title:    "Quiet thread",
+		Body:     "No email configured.",
+		PostDate: pacific.Today(),
+	}
+	if err := mem.CreatePost(context.Background(), noEmailRoot); err != nil {
+		t.Fatal(err)
+	}
+	rec = postForm(handler, "/posts", url.Values{
+		"_csrf":     {adminCSRF},
+		"parent_id": {noEmailRoot.ID},
+		"body":      {"This should not send."},
+	}, adminCookies)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("no-email reply status = %d: %s", rec.Code, rec.Body.String())
+	}
+	time.Sleep(50 * time.Millisecond)
+	if recording.Len() != 2 {
+		t.Fatalf("self, edit, or no-email action sent a notification: %+v", recording.Snapshot())
+	}
+}
+
 func TestQuestionPageRendersNestedPostControls(t *testing.T) {
 	t.Parallel()
 
@@ -345,6 +468,19 @@ func TestQuestionPageRendersNestedPostControls(t *testing.T) {
 	}
 }
 
+func waitForMail(t *testing.T, recording *mail.Recording, want int) []mail.PostReply {
+	t.Helper()
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if recording.Len() >= want {
+			return recording.Snapshot()
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	t.Fatalf("recorded %d notifications, want %d", recording.Len(), want)
+	return nil
+}
+
 func csrfForCookies(t *testing.T, handler http.Handler, cookies []*http.Cookie) string {
 	t.Helper()
 	req := httptest.NewRequest(http.MethodGet, "/submit", nil)