feat(notify): bundled notifications, task watchers, daily summary

- Per-user routing (own mail / own Discord webhook) for task alarms
  (sent at once), shares, completed shared tasks and watched tasks;
  settings as JSON on the user (GET /auth/me, PATCH /auth/settings).
- Everything but alarms is queued and sent as one message per user
  after NOTIFY_DIGEST_MINUTES (default 5); mails now multipart with an
  HTML version, Discord as one embed. Three failed attempts drop it.
- Own changes never notify the actor, except on watched tasks when
  asked for; "paused" stops everything including alarms.
- Task watchers: POST/DELETE /tasks/{id}/watch, isWatching on tasks.
- Daily summary at a chosen time: overdue, today, tomorrow, plus the
  count of the next 7 days; nothing due, nothing sent.
- Migration 7; jobs do not run in the demo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/mail.go

@@ -117,7 +117,7 @@ func senderAddress(raw, name string) (envelope, header string, err error) {
 
 // buildMessage creates an RFC 5322 message: Date and Message-ID headers, RFC 2047 encoded subject
 // (umlauts, emoji) and a quoted-printable UTF-8 body with CRLF line endings.
-func buildMessage(from, to, subject, body string, now time.Time) []byte {
+func buildMessage(from, to, subject, body, html string, now time.Time) []byte {
 	idBytes := make([]byte, 12)
 	_, _ = rand.Read(idBytes)
 	domain := ehloName(from)
@@ -129,12 +129,26 @@ func buildMessage(from, to, subject, body string, now time.Time) []byte {
 	fmt.Fprintf(&buf, "Date: %s\r\n", now.Format(time.RFC1123Z))
 	fmt.Fprintf(&buf, "Message-ID: <%s@%s>\r\n", hex.EncodeToString(idBytes), domain)
 	buf.WriteString("MIME-Version: 1.0\r\n")
-	buf.WriteString("Content-Type: text/plain; charset=\"utf-8\"\r\n")
-	buf.WriteString("Content-Transfer-Encoding: quoted-printable\r\n\r\n")
-	qp := quotedprintable.NewWriter(&buf)
-	_, _ = qp.Write([]byte(strings.ReplaceAll(strings.ReplaceAll(body, "\r\n", "\n"), "\n", "\r\n")))
-	_ = qp.Close()
-	buf.WriteString("\r\n")
+	part := func(contentType, text string) {
+		fmt.Fprintf(&buf, "Content-Type: %s; charset=\"utf-8\"\r\n", contentType)
+		buf.WriteString("Content-Transfer-Encoding: quoted-printable\r\n\r\n")
+		qp := quotedprintable.NewWriter(&buf)
+		_, _ = qp.Write([]byte(strings.ReplaceAll(strings.ReplaceAll(text, "\r\n", "\n"), "\n", "\r\n")))
+		_ = qp.Close()
+		buf.WriteString("\r\n")
+	}
+	if html == "" {
+		part("text/plain", body)
+		return buf.Bytes()
+	}
+	// Text for simple clients, HTML for the rest
+	boundary := "ms-" + hex.EncodeToString(idBytes)
+	fmt.Fprintf(&buf, "Content-Type: multipart/alternative; boundary=\"%s\"\r\n\r\n", boundary)
+	fmt.Fprintf(&buf, "--%s\r\n", boundary)
+	part("text/plain", body)
+	fmt.Fprintf(&buf, "--%s\r\n", boundary)
+	part("text/html", html)
+	fmt.Fprintf(&buf, "--%s--\r\n", boundary)
 	return buf.Bytes()
 }
 
@@ -199,14 +213,14 @@ func authenticate(client *smtp.Client, cfg smtpSettings) error {
 
 // sendMail sends one mail. If AUTH PLAIN is refused and the server also offers LOGIN, it reconnects once
 // and tries LOGIN; a 451 on both points to the mail server's SASL backend.
-func sendMail(cfg smtpSettings, to, subject, body string) error {
-	err := sendMailOnce(cfg, to, subject, body)
+func sendMail(cfg smtpSettings, to, subject, body, html string) error {
+	err := sendMailOnce(cfg, to, subject, body, html)
 	var first *authAttemptError
 	if cfg.authMech != "" || !errors.As(err, &first) || first.mech != "PLAIN" || !first.loginOffered {
 		return err
 	}
 	cfg.authMech = "LOGIN"
-	retryErr := sendMailOnce(cfg, to, subject, body)
+	retryErr := sendMailOnce(cfg, to, subject, body, html)
 	var second *authAttemptError
 	if retryErr == nil || !errors.As(retryErr, &second) {
 		return retryErr
@@ -220,7 +234,7 @@ func sendMail(cfg smtpSettings, to, subject, body string) error {
 
 // sendMailOnce runs the SMTP dialogue step by step instead of smtp.SendMail, so it can introduce itself
 // properly, honour SkipVerify and report the exact step a server refuses.
-func sendMailOnce(cfg smtpSettings, to, subject, body string) error {
+func sendMailOnce(cfg smtpSettings, to, subject, body, html string) error {
 	fromAddr, fromHeader, err := senderAddress(cfg.From, cfg.FromName)
 	if err != nil {
 		return &smtpStepError{"Absender", err}
@@ -278,7 +292,7 @@ func sendMailOnce(cfg smtpSettings, to, subject, body string) error {
 	if err != nil {
 		return &smtpStepError{"DATA", err}
 	}
-	if _, err := wc.Write(buildMessage(fromHeader, toHeader, subject, body, time.Now())); err != nil {
+	if _, err := wc.Write(buildMessage(fromHeader, toHeader, subject, body, html, time.Now())); err != nil {
 		_ = wc.Close()
 		return &smtpStepError{"DATA", err}
 	}