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/internal/ratelimit/ipwindow.go

@@ -0,0 +1,49 @@
+package ratelimit
+
+import (
+	"sync"
+	"time"
+)
+
+// IPWindow limits how often the same IP may perform an action.
+type IPWindow struct {
+	mu     sync.Mutex
+	hits   map[string][]time.Time
+	max    int
+	window time.Duration
+}
+
+func NewIPWindow(max int, window time.Duration) *IPWindow {
+	return &IPWindow{
+		hits:   make(map[string][]time.Time),
+		max:    max,
+		window: window,
+	}
+}
+
+// Allow returns false if this IP has exceeded the limit.
+func (w *IPWindow) Allow(ip string) bool {
+	if ip == "" {
+		ip = "unknown"
+	}
+	now := time.Now()
+	cutoff := now.Add(-w.window)
+
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	ts := w.hits[ip]
+	kept := ts[:0]
+	for _, t := range ts {
+		if t.After(cutoff) {
+			kept = append(kept, t)
+		}
+	}
+	if len(kept) >= w.max {
+		w.hits[ip] = kept
+		return false
+	}
+	kept = append(kept, now)
+	w.hits[ip] = kept
+	return true
+}