docs: add per-tool API documentation and fix parity gaps

- Create 53 per-tool VitePress documentation pages with accurate
  parameters from Zod schemas, example requests, and response formats
- Add root llms.txt for LLM-friendly repo browsing
- Fix OpenAPI spec: add auth and 422 error schemas to
  edit-metadata/inspect and strip-metadata/inspect sub-routes
- Fix tool count inconsistency (52 -> 53) across landing site,
  e2e tests, and local docs
- Rename color-adjustments.ts to adjust-colors.ts to match tool ID
- Update VitePress sidebar with all 8 tool categories and top nav

apps/docs/tools/watermark-image.md

@@ -0,0 +1,58 @@
+---
+description: Overlay a logo or image as a watermark with configurable position, opacity, and scale.
+---
+
+# Image Watermark
+
+Overlay a logo or secondary image as a watermark on a base image. The watermark is scaled relative to the base image width and positioned at a corner or center.
+
+## API Endpoint
+
+`POST /api/v1/tools/watermark-image`
+
+Accepts multipart form data with **two** image files and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| position | string | No | `"bottom-right"` | Watermark placement: `center`, `top-left`, `top-right`, `bottom-left`, `bottom-right` |
+| opacity | number | No | `50` | Watermark opacity percentage (0 to 100) |
+| scale | number | No | `25` | Watermark width as percentage of main image width (1 to 100) |
+
+### File Fields
+
+| Field Name | Required | Description |
+|------------|----------|-------------|
+| file | Yes | The main/base image |
+| watermark | Yes | The watermark/logo image |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/watermark-image \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@photo.jpg" \
+  -F "watermark=@logo.png" \
+  -F 'settings={"position": "bottom-right", "opacity": 60, "scale": 20}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/photo.jpg",
+  "originalSize": 2450000,
+  "processedSize": 2520000
+}
+```
+
+## Notes
+
+- Both images are validated and decoded (HEIC, RAW, PSD, SVG supported).
+- The watermark is resized proportionally so its width equals `scale`% of the main image width.
+- Opacity is applied via an alpha mask composited with `dest-in` blending.
+- Corner positions use a 20px padding from the image edge.
+- If the watermark image has transparency (e.g., a PNG logo), it is preserved during compositing.
+- EXIF orientation is auto-applied on both images before processing.