docs: organize tool pages by modality under /tools/<modality>/

Move all 157 tool pages into image/video/audio/pdf/data subfolders so URLs read /tools/<modality>/<id> (e.g. /tools/image/crop). Nest the image sub-categories under an Image group in the sidebar so the nav reads by modality. Add public/_redirects (301, clean + .html forms) mapping every old flat /tools/<id> URL to its new path so inbound links keep working. Rewrite all internal /tools links. Verified with a clean docs build (no dead links).

apps/docs/tools/image/duotone.md

@@ -0,0 +1,47 @@
+---
+description: Apply a two-color duotone effect with custom shadow and highlight colors.
+---
+
+# Duotone
+
+Apply a two-color duotone effect to an image. The image is converted to grayscale, then mapped to a gradient between the shadow color (dark tones) and the highlight color (bright tones).
+
+## API Endpoint
+
+`POST /api/v1/tools/image/duotone`
+
+Accepts multipart form data with an image file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| shadow | string | No | `"#1e3a8a"` | Shadow hex color (applied to dark tones) |
+| highlight | string | No | `"#fbbf24"` | Highlight hex color (applied to bright tones) |
+| intensity | integer | No | `100` | Effect intensity (0-100); 0 returns the original, 100 applies the full duotone |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/image/duotone \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@photo.jpg" \
+  -F 'settings={"shadow": "#0f172a", "highlight": "#f97316", "intensity": 80}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/photo.jpg",
+  "originalSize": 2450000,
+  "processedSize": 1870000
+}
+```
+
+## Notes
+
+- Output format matches the input format. HEIC, RAW, PSD, and SVG inputs are automatically decoded before processing.
+- An `intensity` of less than 100 blends the duotone result with the original image, allowing for subtler effects.
+- Popular duotone combinations include navy/gold, teal/coral, and purple/pink.