Add contact antispam and fix gallery video playback.

English-only messages, rate limiting, min fill time, and normalized email
validation; improve modal video serving with posters, correct MIME types, and
no gzip on gallery media.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/static/style.css

@@ -524,16 +524,49 @@ main {
   background: #000;
 }
 
+.modal-video {
+  position: relative;
+  width: 100%;
+}
+
 .modal-figure img,
 .modal-figure video {
   display: block;
   width: 100%;
   max-height: 75vh;
+  min-height: 12rem;
   object-fit: contain;
   border-radius: 8px;
   background: #000;
 }
 
+.modal-figure video.video-broken {
+  display: none;
+}
+
+.video-unavailable {
+  margin: 0;
+  padding: 1.25rem;
+  text-align: center;
+  color: var(--text-muted);
+  background: #000;
+  border-radius: 8px;
+  min-height: 12rem;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 0.75rem;
+}
+
+.video-unavailable[hidden] {
+  display: none !important;
+}
+
+.video-unavailable a {
+  color: var(--accent);
+}
+
 .modal-figure figcaption {
   margin-top: 0.75rem;
   color: var(--text-muted);