Add per-IP rate limiting and Unix socket support for gRPC admin API

Rate limiting: per-source-IP connection rate limiter in the firewall layer
with configurable limit and sliding window. Blocklisted IPs are rejected
before rate limit evaluation to avoid wasting quota. Unix socket: the gRPC
admin API can now listen on a Unix domain socket (no TLS required), secured
by file permissions (0600), as a simpler alternative for local-only access.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/sni/sni.go

@@ -165,6 +165,9 @@ func parseServerNameExtension(data []byte) (string, error) {
 		}
 
 		if nameType == 0x00 { // hostname
+			if nameLen > 255 {
+				return "", fmt.Errorf("SNI hostname exceeds 255 bytes")
+			}
 			return strings.ToLower(string(data[:nameLen])), nil
 		}