feat(mail): add site URL handling and notification headers

* include site URL in Mailer for absolute URLs
* add header method for email notifications

server/internal/mail/mail.go

@@ -14,11 +14,31 @@ import (
 )
 
 type Mailer struct {
-	cfg config.EmailConfig
+	cfg     config.EmailConfig
+	siteURL string
 }
 
-func New(cfg config.EmailConfig) *Mailer {
-	return &Mailer{cfg: cfg}
+func New(cfg config.EmailConfig, siteURL string) *Mailer {
+	return &Mailer{cfg: cfg, siteURL: strings.TrimSuffix(siteURL, "/")}
+}
+
+// absoluteURL turns a site-relative path (e.g. "/uploads/1/foo.jpg") into a
+// full URL so it's clickable straight from an email client.
+func (m *Mailer) absoluteURL(path string) string {
+	if m.siteURL == "" {
+		return path
+	}
+	return m.siteURL + path
+}
+
+// header prefixes every notification with a one-line explainer of where it
+// came from and what triggered it, so it's never a mystery in an inbox.
+func (m *Mailer) header(event string) string {
+	site := m.siteURL
+	if site == "" {
+		site = "the wedding site"
+	}
+	return fmt.Sprintf("This is an automated email that gets triggered for %s when %s.\n\n", site, event)
 }
 
 // newMessageID builds an RFC 5322 Message-ID, scoped to the sender's own
@@ -93,7 +113,7 @@ func (m *Mailer) SendRSVP(r store.RSVP, allGuests []store.Guest) error {
 		fmt.Fprintf(&b, "\n%d adult(s), %d child(ren) total\n", totalAdults, totalChildren)
 	}
 
-	return m.send(fmt.Sprintf("RSVP from %s", joined), b.String())
+	return m.send(fmt.Sprintf("RSVP from %s", joined), m.header("an RSVP is made")+b.String())
 }
 
 func (m *Mailer) SendPhotoUpload(u store.PhotoUpload, files []store.PhotoFile) error {
@@ -110,7 +130,7 @@ func (m *Mailer) SendPhotoUpload(u store.PhotoUpload, files []store.PhotoFile) e
 	}
 	b.WriteString("\nPhotos:\n")
 	for _, f := range files {
-		fmt.Fprintf(&b, "- %s\n", f.URL)
+		fmt.Fprintf(&b, "- %s\n", m.absoluteURL(f.URL))
 	}
-	return m.send(fmt.Sprintf("%s uploaded photos", u.Name), b.String())
+	return m.send(fmt.Sprintf("%s uploaded photos", u.Name), m.header("photos are uploaded")+b.String())
 }