Per-user intro versioning + admin pending-delivery view

Replaces "clear intro after first send", which dropped the message
for any subscriber still queued on a slower cadence.

- Each non-empty admin save bumps a per-locale version counter on
  the tree. The dispatcher includes the intro only for recipients
  whose last-seen version is behind, then advances their watermark
  after a successful send. Webtrees users get a per-user watermark;
  external addresses share one tree-level watermark. Re-saving the
  same text is a no-op.
- The preferences page now shows delivery progress per locale:
  "Delivered to X of Y subscriber(s)" plus a collapsible Pending
  list with name + email of each subscriber who hasn't received
  the current intro yet, and a single "External recipients (N)"
  row when the external watermark is behind.
- README rewritten to reflect every feature shipped since the
  initial commit (BockenTheme skin, embedded avatars, relationship
  labels, kin-distance filter, per-user cadence, bilingual subject
  prefix, locale-aware subject date, SiteUser as From, three
  subscriber sources, Markdown intro with personalisation tokens).

src/Module.php

@@ -173,9 +173,16 @@ class Module extends AbstractModule implements ModuleCustomInterface, ModuleConf
                 'Intro paragraph for the next email'          => 'Einleitungsabsatz für die nächste E-Mail',
                 'Shown once, above the upcoming events. Cleared automatically after the next successful send.'
                                                               => 'Wird einmalig über den anstehenden Ereignissen angezeigt. Wird nach dem nächsten erfolgreichen Versand automatisch geleert.',
+                'Delivered once to every subscriber on their own cadence. Edit and save the text to send a new intro to everyone.'
+                                                              => 'Wird jedem Abonnenten einmalig in seinem eigenen Versandrhythmus zugestellt. Text bearbeiten und speichern, um eine neue Einleitung an alle zu versenden.',
                 'Personalisation tokens:'                     => 'Personalisierungs-Platzhalter:',
                 'Formatted as Markdown — e.g. %1$s for emphasis, %2$s for a link.'
                                                               => 'Formatiert als Markdown — z. B. %1$s für Hervorhebung, %2$s für einen Link.',
+                'Delivered to all %d subscriber(s).'          => 'An alle %d Abonnenten zugestellt.',
+                'Delivered to %1$d of %2$d subscriber(s).'    => 'An %1$d von %2$d Abonnenten zugestellt.',
+                'Pending'                                     => 'Ausstehend',
+                'External recipients (%d)'                    => 'Externe Empfänger (%d)',
+                'Save to schedule delivery.'                  => 'Speichern, um die Zustellung zu starten.',
             ],
             'nl' => [
                 'Email Newsletter'                            => 'E-mailnieuwsbrief',
@@ -293,7 +300,17 @@ class Module extends AbstractModule implements ModuleCustomInterface, ModuleConf
                 $intro = Validator::parsedBody($request)
                     ->string('intro-' . $id . '-' . $code, '');
 
+                // Bump the version only when the saved text actually
+                // changed AND is non-empty. That makes "save the same
+                // intro again" a no-op (no resends), while saving a
+                // new non-empty paragraph re-delivers it to every
+                // subscriber on their own cadence.
+                $previous = Configuration::introForLocale($this, $tree, $code);
                 Configuration::setIntroForLocale($this, $tree, $code, $intro);
+
+                if ($intro !== '' && $intro !== $previous) {
+                    Configuration::bumpIntroVersion($this, $tree, $code);
+                }
             }
 
             // Per-user subscription toggles. A users-roster marker is