Generate real resized thumbnails for image attachments (#98)

The attachment grid's <img> thumbnail pointed at the same
download_attachment.php URL as the full-size original, so previewing a
multi-MB photo attachment cost a full multi-MB download just to render a
small grid preview. loading="lazy" only deferred off-screen images; it
never reduced per-image transfer size.

Generate a resized JPEG thumbnail (longest side capped at 300px) via GD
at upload time, from the same metadata-stripped image stripImageMetadata()
already produces, reusing its decompression-bomb guard (~40MP decode cap).
Store the thumbnail's filename in a new nullable ticket_attachments.
thumbnail_filename column (migration 005); NULL means no thumbnail exists
(non-image, GD unavailable, or an attachment predating this change) and
callers fall back to the full-size original.

download_attachment.php serves the thumbnail when requested via
?thumb=1 and one exists, falling back to the original otherwise. The
attachments grid now requests thumb=1 for its <img> preview; the
lightbox link is unchanged and still opens the full-size original.
delete_attachment.php removes the thumbnail file alongside the original,
and cleanup_orphan_uploads.php's orphan lookup now also matches
thumbnail_filename so generated thumbnails aren't swept up as orphans.

Verified against real MariaDB + GD: a 1600x1200 test JPEG produced a
300x225 thumbnail at ~1.8KB vs. the 52KB original (~29x smaller);
confirmed the serving logic picks the thumbnail for image attachments
with one, falls back to the original for a non-image attachment even
when thumb=1 is requested, and that the updated orphan-cleanup lookup
matches both the original and thumbnail filename (and correctly finds
neither for an unrelated filename).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117oBw2jN4kALYeS8HPq4zV

api/download_attachment.php

@@ -69,9 +69,21 @@ try {
 
     $conn->close();
 
+    // Serve the resized preview thumbnail instead of the full-size original
+    // when requested and one was actually generated at upload time; falls
+    // through to the full original otherwise (older attachments predating
+    // thumbnail generation, non-images, or a GD failure at upload time).
+    $wantsThumb = isset($_GET['thumb']) && $_GET['thumb'] === '1';
+    $servedFilename = $attachment['filename'];
+    $servedMimeType = $attachment['mime_type'];
+    if ($wantsThumb && !empty($attachment['thumbnail_filename'])) {
+        $servedFilename = $attachment['thumbnail_filename'];
+        $servedMimeType = 'image/jpeg';
+    }
+
     // Build file path
     $uploadDir = $GLOBALS['config']['UPLOAD_DIR'] ?? dirname(__DIR__) . '/uploads';
-    $filePath = $uploadDir . '/' . $attachment['ticket_id'] . '/' . $attachment['filename'];
+    $filePath = $uploadDir . '/' . $attachment['ticket_id'] . '/' . $servedFilename;
 
     // Security: Verify the resolved path is within the uploads directory (prevent path traversal)
     $realUploadDir = realpath($uploadDir);
@@ -100,7 +112,7 @@ try {
     $inlineTypes = ['image/jpeg', 'image/png', 'image/gif', 'image/webp', 'application/pdf', 'text/plain'];
 
     // Set headers
-    $disposition = ($inline && in_array($attachment['mime_type'], $inlineTypes)) ? 'inline' : 'attachment';
+    $disposition = ($inline && in_array($servedMimeType, $inlineTypes)) ? 'inline' : 'attachment';
 
     // Sanitize filename for Content-Disposition
     $safeFilename = preg_replace('/[^\w\s\-\.]/', '_', $attachment['original_filename']);
@@ -138,7 +150,7 @@ try {
     $rangeLength = $rangeEnd - $rangeStart + 1;
 
     header('Accept-Ranges: bytes');
-    header('Content-Type: ' . $attachment['mime_type']);
+    header('Content-Type: ' . $servedMimeType);
     header('Content-Disposition: ' . $disposition . '; filename="' . $safeFilename . '"');
     header('Cache-Control: private, max-age=3600');
     header('X-Content-Type-Options: nosniff');