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/pdf/crop-pdf.md

@@ -0,0 +1,45 @@
+---
+description: Crop all pages of a PDF with a uniform margin.
+---
+
+# Crop PDF
+
+Crop all pages of a PDF by applying a uniform margin, trimming content from each edge equally.
+
+## API Endpoint
+
+`POST /api/v1/tools/pdf/crop-pdf`
+
+Accepts multipart form data with a PDF file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| margin | number | No | `20` | Uniform crop margin in points (0-2000) |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/pdf/crop-pdf \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@document.pdf" \
+  -F 'settings={"margin": 50}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/document.pdf",
+  "originalSize": 2450000,
+  "processedSize": 2440000
+}
+```
+
+## Notes
+
+- The margin value is in PDF points (1 point = 1/72 inch).
+- The same margin is applied to all four edges of every page.
+- A margin of `0` removes all existing crop margins, showing the full media box.