fix: replace enhancement pipeline with CLAHE + normalise + gamma

CLAHE provides adaptive local contrast, normalise stretches the
histogram, and gamma adjusts exposure perceptually. Replaces the old
modulate/linear pipeline that compounded errors and darkened images.
Preset multipliers now include clahe and normalise entries.

Key fixes beyond the spec:
- maxSlope rounded to integer (Sharp requirement)
- White balance uses linear() instead of recomb() to avoid float-cast
  that breaks CLAHE in the libvips pipeline
- CLAHE tile size adapts to image dimensions (1x1 for tiny images)
- Gamma clamped to Sharp's valid range (1.0-3.0)
- Normalise lower/upper correctly mapped to percentile cutoffs

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

@@ -33,6 +33,7 @@ async function processImageEnhancement(
 ) {
   const outputFormat = await resolveOutputFormat(inputBuffer, filename);
   const analysis = await analyzeImage(inputBuffer);
+  const meta = await sharp(inputBuffer).metadata();
 
   let image = sharp(inputBuffer);
   image = applyCorrections(
@@ -41,6 +42,7 @@ async function processImageEnhancement(
     settings.mode,
     settings.intensity,
     settings.corrections,
+    { width: meta.width ?? 1, height: meta.height ?? 1 },
   );
 
   const buffer = await image