fix: use FileChannel.transferTo for zero-copy file serving

Replaced RandomAccessFile + manual 8KB buffer loop with
FileChannel.transferTo() which uses OS sendfile — zero copy
from disk cache to network socket. Eliminates Java heap churn
and matches the old Files.copy() performance while supporting
offset/length for HTTP Range requests.

src/main/java/com/dostalgia/StaticResource.java

@@ -161,24 +161,15 @@ public class StaticResource {
 
     /**
      * Streaming file output that avoids loading the whole file into memory.
-     * Supports partial reads (seek + limit) for HTTP Range requests.
+     * Supports partial reads (offset + length) for HTTP Range requests.
+     * Uses FileChannel.transferTo() for zero-copy sendfile semantics.
      */
     private record FileStream(Path file, long offset, long length) implements StreamingOutput {
         @Override
         public void write(OutputStream output) throws IOException {
-            try (var raf = new RandomAccessFile(file.toFile(), "r");
-                 var channel = raf.getChannel()) {
-                if (offset > 0) {
-                    raf.seek(offset);
-                }
-                long remaining = length;
-                byte[] buf = new byte[8192];
-                while (remaining > 0) {
-                    int toRead = (int) Math.min(buf.length, remaining);
-                    int read = raf.read(buf, 0, toRead);
-                    if (read < 0) break;
-                    output.write(buf, 0, read);
-                    remaining -= read;
+            try (FileChannel ch = FileChannel.open(file, StandardOpenOption.READ)) {
+                if (length > 0) {
+                    ch.transferTo(offset, length, output);
                 }
             }
         }