fix: CLAHE tile size and alpha channel corruption in image enhancement

CLAHE width/height is tile size in pixels, not tile count. A 3px tile on
a 992x1088 image created ~330x360 independent histogram regions, producing
crosshatch/etching artifacts. Now uses image_dimension/8 (clamped 8-256)
for ~8 tiles per axis. Also strips alpha before enhancement and re-joins
after to prevent CLAHE/normalise/linear from corrupting transparency.

apps/api/src/routes/tools/image-enhancement.ts

@@ -41,8 +41,18 @@ async function processImageEnhancement(
   const outputFormat = await resolveOutputFormat(inputBuffer, filename);
   const analysis = await analyzeImage(inputBuffer);
   const meta = await sharp(inputBuffer).metadata();
+  const hasAlpha = meta.hasAlpha === true;
+
+  let alphaBuffer: Buffer | undefined;
+  if (hasAlpha) {
+    alphaBuffer = await sharp(inputBuffer).extractChannel(3).toBuffer();
+  }
 
   let image = sharp(inputBuffer);
+  if (hasAlpha) {
+    image = image.removeAlpha();
+  }
+
   image = applyCorrections(
     image,
     analysis.corrections,
@@ -56,6 +66,13 @@ async function processImageEnhancement(
     .toFormat(outputFormat.format, { quality: outputFormat.quality })
     .toBuffer();
 
+  if (alphaBuffer) {
+    buffer = await sharp(buffer)
+      .joinChannel(alphaBuffer)
+      .toFormat(outputFormat.format, { quality: outputFormat.quality })
+      .toBuffer();
+  }
+
   if (settings.deepEnhance && isToolInstalled("noise-removal")) {
     try {
       const jobId = randomUUID();