Feature: Walk-Einladungen und RSVP-System

Gassi-Treffen bekommen ein vollständiges Einladungs- und RSVP-System:
- Neue Tabelle walk_invitations (walk_id, user_id, status, Zeitstempel)
- Backend: /invite-candidates, /invite, /rsvp, /participants Endpoints
- Push-Notification beim Einladen
- Frontend: RSVP-Buttons (Zusagen/Vielleicht/Absagen), Teilnehmerliste
  mit Avatar-Initialen und farbkodierten RSVP-Badges, Einladen-Modal
- SW by-v205, APP_VER 173

backend/static/css/components.css

@@ -2602,6 +2602,112 @@ html.modal-open {
 .walks-participant-name { font-weight: var(--weight-semibold); }
 .walks-participant-dogs { color: var(--c-text-secondary); }
 
+/* Walk-Einladungen + RSVP */
+.walks-invitation-row,
+.walks-invite-row {
+  display:       flex;
+  align-items:   center;
+  gap:           var(--space-3);
+  padding:       var(--space-2) 0;
+  border-bottom: 1px solid var(--c-border);
+  font-size:     var(--text-sm);
+}
+.walks-invitation-row:last-child,
+.walks-invite-row:last-child { border-bottom: none; }
+
+.walks-inv-avatar {
+  width:           36px;
+  height:          36px;
+  border-radius:   50%;
+  background:      var(--c-primary);
+  color:           #fff;
+  display:         flex;
+  align-items:     center;
+  justify-content: center;
+  font-size:       var(--text-xs);
+  font-weight:     var(--weight-semibold);
+  flex-shrink:     0;
+}
+.walks-inv-avatar--sm {
+  width:  28px;
+  height: 28px;
+}
+.walks-inv-info { flex: 1; min-width: 0; }
+.walks-inv-name {
+  font-weight: var(--weight-semibold);
+  white-space: nowrap;
+  overflow:    hidden;
+  text-overflow: ellipsis;
+}
+.walks-inv-hunde {
+  color:     var(--c-text-secondary);
+  font-size: var(--text-xs);
+}
+.walks-inv-badge { flex-shrink: 0; }
+
+/* RSVP-Badges */
+.walks-rsvp-badge {
+  display:       inline-block;
+  padding:       2px 8px;
+  border-radius: var(--radius-full);
+  font-size:     var(--text-xs);
+  font-weight:   var(--weight-semibold);
+}
+.walks-rsvp--yes     { background: #d1fae5; color: #065f46; }
+.walks-rsvp--maybe   { background: #fef3c7; color: #92400e; }
+.walks-rsvp--no      { background: #fee2e2; color: #991b1b; }
+.walks-rsvp--invited { background: var(--c-surface-2); color: var(--c-text-secondary); }
+
+/* RSVP-Section im Detail-Modal */
+.walks-rsvp-section {
+  margin:        var(--space-3) 0;
+  padding:       var(--space-3);
+  background:    var(--c-surface-2);
+  border-radius: var(--radius-md);
+}
+.walks-rsvp-buttons {
+  display:   flex;
+  gap:       var(--space-2);
+  flex-wrap: wrap;
+  margin-top: var(--space-2);
+}
+.walks-rsvp-btn {
+  flex:          1;
+  min-width:     80px;
+  padding:       var(--space-2) var(--space-2);
+  border:        2px solid var(--c-border);
+  border-radius: var(--radius-md);
+  background:    var(--c-surface);
+  color:         var(--c-text);
+  font-size:     var(--text-sm);
+  font-weight:   var(--weight-semibold);
+  cursor:        pointer;
+  transition:    all 0.15s;
+  display:       flex;
+  align-items:   center;
+  justify-content: center;
+  gap:           var(--space-1);
+}
+.walks-rsvp-btn:hover {
+  border-color: var(--c-primary);
+  background:   var(--c-primary-bg, rgba(196,132,58,0.08));
+}
+.walks-rsvp-btn.active[data-rsvp="yes"] {
+  border-color: #10b981;
+  background:   #d1fae5;
+  color:        #065f46;
+}
+.walks-rsvp-btn.active[data-rsvp="maybe"] {
+  border-color: #f59e0b;
+  background:   #fef3c7;
+  color:        #92400e;
+}
+.walks-rsvp-btn.active[data-rsvp="no"] {
+  border-color: #ef4444;
+  background:   #fee2e2;
+  color:        #991b1b;
+}
+
 /* ------------------------------------------------------------
    EVENTS (events.js)
    ------------------------------------------------------------ */