fix: security hardening for middleware, crypto, and backup handlers

- Remove CORS origin reflection (SEC-C1 CRITICAL)
- Add Content-Security-Policy header (SEC-H2)
- Fix rate limiter memory leak with periodic stale IP cleanup (SEC-H5)
- Enforce minimum 32-char ENCRYPTION_KEY (SEC-H4)
- Validate backup type against allowlist (SEC-M6)
- Fix backup download path traversal with path containment check (SEC-C2 CRITICAL)

internal/crypto/crypto.go

@@ -28,6 +28,9 @@ func KeyFromEnv() ([32]byte, error) {
 	if raw == "" {
 		return [32]byte{}, ErrNoKey
 	}
+	if len(raw) < 32 {
+		return [32]byte{}, fmt.Errorf("ENCRYPTION_KEY must be at least 32 characters long (got %d)", len(raw))
+	}
 	return DeriveKey(raw), nil
 }