feat(api): notifications inbox and per-user settings endpoints (#5339)

M489 step 2.

- GET /api/me/notifications?limit&before: newest first, keyset-paged on
  (created_at, id) with an opaque cursor, plus the unread count.
- GET /api/me/notifications/unread-count: the badge's cheap call.
- POST /api/me/notifications/{id}/read and /read-all. Mark-read is
  idempotent; another user's id is a 404, the same as a malformed one.
- GET/PUT /api/me/notification-settings: every kind the caller can receive
  (admin kinds only for admins) with inbox/phone/email. PUT is partial, so an
  offline replay sends only what was touched, and a batch with any invalid
  change applies nothing. The response says whether email can be delivered
  at all: no address on file, or SMTP not configured. A failed SMTP config
  read is a 500, not "not configured".

notifications.Render turns kind + payload into title, body and link on the
server, so the web inbox, the Android inbox, the phone's shade and the email
digest all say the same thing. mailer.Configured lifts Send's readiness
check out so settings can report it.

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

internal/mailer/mailer.go

@@ -57,7 +57,7 @@ func (s *SMTPSender) Send(ctx context.Context, to, subject, textBody, htmlBody s
 	if err != nil {
 		return fmt.Errorf("mailer: load config: %w", err)
 	}
-	if !cfg.Enabled || cfg.Host == "" || cfg.FromAddress == "" {
+	if !Configured(cfg) {
 		return ErrNotConfigured
 	}
 
@@ -86,6 +86,13 @@ func (s *SMTPSender) Send(ctx context.Context, to, subject, textBody, htmlBody s
 	return nil
 }
 
+// Configured reports whether cfg can send at all: enabled, with a host and a
+// from address. Send refuses with ErrNotConfigured otherwise, and settings
+// screens use it to say why email is unavailable before anyone tries.
+func Configured(cfg dbq.SmtpConfig) bool {
+	return cfg.Enabled && cfg.Host != "" && cfg.FromAddress != ""
+}
+
 // sendMail wraps net/smtp's SendMail with optional TLS verification.
 // Mostly identical to smtp.SendMail but explicitly handles the
 // use_tls flag.