docs: add transparency-fixer endpoint documentation

apps/docs/api/ai.md

@@ -2,7 +2,7 @@
 
 The `@snapotter/ai` package bridges Node.js to a **persistent Python sidecar** for all ML operations. The dispatcher process stays alive between requests for fast warm-start performance. GPU is auto-detected at startup and used when available.
 
-14 AI tool routes. All models run locally - no internet required after initial model download.
+15 AI tool routes. All models run locally - no internet required after initial model download.
 
 ## Architecture
 
@@ -26,6 +26,7 @@ Node.js Tool Route
       ├─ noise_removal.py    (tiered denoising)
       ├─ red_eye_removal.py  (landmark + color analysis)
       ├─ restore.py          (scratch repair + enhancement + denoising)
+      ├─ transparency_fix.py (BiRefNet HR-matting + defringe)
       └─ seam_carving        (Go caire binary - not Python)
 ```
 
@@ -255,3 +256,27 @@ Intelligently resizes images by removing or adding low-energy seams, preserving
 | `square` | boolean | false | Force square output |
 
 Max input edge before auto-downscaling: **1200 px**.
+
+## Transparency Fixer
+
+**Function:** `fixTransparency`  
+**Tool route:** `transparency-fixer`  
+**Model:** BiRefNet HR-matting (2048x2048 resolution)
+
+Fixes "fake transparent" PNGs where the background was removed but left behind fringing, halos, or semi-transparent artifacts. Uses BiRefNet's high-resolution matting model to produce a clean alpha channel, then applies configurable defringe processing to remove color contamination along edges.
+
+**OOM fallback chain:** If BiRefNet HR-matting exceeds available memory, the tool automatically falls back to `birefnet-general`, then to `u2net`.
+
+**Feature bundle:** Background Removal (shared with Remove Background and Passport Photo).
+
+| Parameter | Type | Default | Description |
+|-----------|------|---------|-------------|
+| `defringe` | number (0-100) | 30 | Edge defringe strength to remove color contamination |
+| `outputFormat` | `"png"` \| `"webp"` | `"png"` | Output image format |
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/transparency-fixer \
+  -H "Authorization: Bearer <token>" \
+  -F "file=@fake-transparent.png" \
+  -F 'settings={"defringe":30,"outputFormat":"png"}'
+```

apps/docs/api/rest.md

@@ -152,6 +152,7 @@ All AI tools run on your hardware (CPU or NVIDIA GPU). No internet required.
 | `restore-photo` | Photo Restoration | Multi-step pipeline | `mode` (auto/light/heavy), `scratchRemoval`, `faceEnhancement`, `fidelity`, `denoise`, `denoiseStrength`, `colorize` |
 | `passport-photo` | Passport Photo | MediaPipe landmarks | `country` (37 countries), `printLayout` (4x6/A4/none), `backgroundColor` |
 | `content-aware-resize` | Content-Aware Resize | Seam carving (caire) | `width`, `height`, `protectFaces`, `blurRadius`, `sobelThreshold`, `square` |
+| `transparency-fixer` | Transparency Fixer | BiRefNet HR-matting | `defringe` (0-100), `outputFormat` (png/webp) |
 
 ### Watermark & Overlay