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/replace-color.md

@@ -0,0 +1,59 @@
+---
+description: Replace a specific color in an image with another color or make it transparent.
+---
+
+# Replace & Invert Color
+
+Replace pixels matching a source color with a target color, or make them transparent. Uses Euclidean distance in RGB space with configurable tolerance for smooth blending at color boundaries.
+
+## API Endpoint
+
+`POST /api/v1/tools/replace-color`
+
+Accepts multipart form data with an image file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| sourceColor | string | No | `"#FF0000"` | Hex color to find (format: `#RRGGBB`) |
+| targetColor | string | No | `"#00FF00"` | Hex color to replace with (format: `#RRGGBB`) |
+| makeTransparent | boolean | No | `false` | Make matching pixels transparent instead of replacing with target color |
+| tolerance | number | No | `30` | Color matching tolerance (0 to 255). Higher values match a wider range of similar colors |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/replace-color \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@photo.jpg" \
+  -F 'settings={"sourceColor": "#FF0000", "targetColor": "#0000FF", "tolerance": 40}'
+```
+
+Make a green background transparent:
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/replace-color \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@greenscreen.png" \
+  -F 'settings={"sourceColor": "#00FF00", "makeTransparent": true, "tolerance": 50}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/photo.png",
+  "originalSize": 2450000,
+  "processedSize": 2100000
+}
+```
+
+## Notes
+
+- Color matching uses Euclidean distance in RGB space, scaled by `tolerance * sqrt(3)`.
+- Replacement blending is proportional to color distance: pixels closer to the source color receive more of the target color, creating smooth transitions.
+- When `makeTransparent` is `true`, the output is forced to PNG (or WebP/AVIF) if the input format does not support alpha channels (e.g., JPEG).
+- A tolerance of 0 matches only the exact source color. Higher values (50+) will match a broader range of similar hues.
+- Output format matches the input format unless transparency is needed and the input format lacks alpha support.