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/upload_attachment.php

@@ -96,6 +96,72 @@ function stripImageMetadata(string $path, string $mimeType): void
     }
 }
 
+/**
+ * Generate a resized preview thumbnail for an uploaded image, saved as a JPEG
+ * alongside the original regardless of source format (a thumbnail is a small
+ * lossy preview, not an archival copy). Longest side capped at
+ * THUMBNAIL_MAX_DIMENSION; images already at or below that size are still
+ * re-encoded (cheap) rather than skipped, so the thumbnail is guaranteed to
+ * be a JPEG the grid can always request the same way.
+ *
+ * Best-effort like stripImageMetadata(): returns null on any failure
+ * (corrupt image, unsupported format, GD unavailable) rather than blocking
+ * the upload, and the caller falls back to serving the full-size original.
+ *
+ * @return string|null Basename of the generated thumbnail file, or null
+ */
+function generateThumbnail(string $path, string $mimeType, string $destDir): ?string
+{
+    if (!extension_loaded('gd')) {
+        return null;
+    }
+
+    // Same decompression-bomb guard as stripImageMetadata().
+    $dims = @getimagesize($path);
+    if ($dims === false) {
+        return null;
+    }
+    [$width, $height] = $dims;
+    if ($width * $height > 40_000_000) { // ~40 MP cap
+        return null;
+    }
+
+    $loaders = [
+        'image/jpeg' => 'imagecreatefromjpeg',
+        'image/png'  => 'imagecreatefrompng',
+        'image/gif'  => 'imagecreatefromgif',
+        'image/webp' => 'imagecreatefromwebp',
+    ];
+    $loader = $loaders[$mimeType] ?? null;
+    if ($loader === null || !function_exists($loader)) {
+        return null;
+    }
+
+    $source = @$loader($path);
+    if ($source === false) {
+        return null;
+    }
+
+    $maxDimension = 300;
+    $scale = min(1.0, $maxDimension / max($width, $height));
+    $thumbWidth  = max(1, (int)round($width * $scale));
+    $thumbHeight = max(1, (int)round($height * $scale));
+
+    $thumb = imagecreatetruecolor($thumbWidth, $thumbHeight);
+    // Flatten transparency onto white — thumbnails are always opaque JPEGs.
+    $white = imagecolorallocate($thumb, 255, 255, 255);
+    imagefill($thumb, 0, 0, $white);
+    imagecopyresampled($thumb, $source, 0, 0, 0, 0, $thumbWidth, $thumbHeight, $width, $height);
+    imagedestroy($source);
+
+    $thumbFilename = pathinfo($path, PATHINFO_FILENAME) . '_thumb.jpg';
+    $thumbPath = rtrim($destDir, '/') . '/' . $thumbFilename;
+    $saved = imagejpeg($thumb, $thumbPath, 80);
+    imagedestroy($thumb);
+
+    return $saved ? $thumbFilename : null;
+}
+
 // Check authentication
 if (!isset($_SESSION['user']) || !isset($_SESSION['user']['user_id'])) {
     ResponseHelper::unauthorized();
@@ -133,6 +199,8 @@ if ($_SERVER['REQUEST_METHOD'] === 'GET') {
         foreach ($attachments as &$att) {
             $att['file_size_formatted'] = AttachmentModel::formatFileSize($att['file_size']);
             $att['icon'] = AttachmentModel::getFileIcon($att['mime_type']);
+            $att['has_thumbnail'] = !empty($att['thumbnail_filename']);
+            unset($att['thumbnail_filename']); // internal storage detail, not needed by the client
         }
 
         ResponseHelper::success([
@@ -278,9 +346,14 @@ if (!move_uploaded_file($file['tmp_name'], $targetPath)) {
     ResponseHelper::serverError('Failed to move uploaded file');
 }
 
-// Strip EXIF/GPS metadata from image uploads before it's ever served back
+// Strip EXIF/GPS metadata from image uploads before it's ever served back,
+// then generate a resized preview thumbnail from the (now metadata-stripped)
+// original so the grid never has to transfer the full-size file just to
+// render a small preview.
+$thumbnailFilename = null;
 if (str_starts_with($mimeType, 'image/')) {
     stripImageMetadata($targetPath, $mimeType);
+    $thumbnailFilename = generateThumbnail($targetPath, $mimeType, $ticketDir);
 }
 
 // Sanitize original filename
@@ -298,12 +371,16 @@ try {
         $originalFilename,
         $file['size'],
         $mimeType,
-        $_SESSION['user']['user_id']
+        $_SESSION['user']['user_id'],
+        $thumbnailFilename
     );
 
     if (!$attachmentId) {
-        // Clean up file if database insert fails
+        // Clean up file (and any thumbnail) if database insert fails
         unlink($targetPath);
+        if ($thumbnailFilename !== null) {
+            @unlink($ticketDir . '/' . $thumbnailFilename);
+        }
         ResponseHelper::serverError('Failed to save attachment record');
     }
 
@@ -330,13 +407,17 @@ try {
         'file_size_formatted' => AttachmentModel::formatFileSize($file['size']),
         'mime_type' => $mimeType,
         'icon' => AttachmentModel::getFileIcon($mimeType),
+        'has_thumbnail' => $thumbnailFilename !== null,
         'uploaded_by' => $_SESSION['user']['display_name'] ?? $_SESSION['user']['username'],
         'uploaded_at' => date('Y-m-d H:i:s')
     ], 'File uploaded successfully');
 } catch (Exception $e) {
-    // Clean up file on error
+    // Clean up file (and any thumbnail) on error
     if (file_exists($targetPath)) {
         unlink($targetPath);
     }
+    if (isset($thumbnailFilename) && $thumbnailFilename !== null) {
+        @unlink($ticketDir . '/' . $thumbnailFilename);
+    }
     ResponseHelper::serverError('Failed to process attachment');
 }