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/watermark-pdf.md

@@ -0,0 +1,55 @@
+---
+description: Add a text watermark to every page of a PDF.
+---
+
+# Watermark PDF
+
+Stamp a text watermark on every page of a PDF with configurable position, size, opacity, and rotation.
+
+## API Endpoint
+
+`POST /api/v1/tools/pdf/watermark-pdf`
+
+Accepts multipart form data with a PDF file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| text | string | Yes | - | Watermark text (1-200 characters) |
+| position | string | No | `"c"` | Placement on the page: `tl`, `tc`, `tr`, `l`, `c`, `r`, `bl`, `bc`, `br` |
+| fontSize | integer | No | `48` | Font size in points (6-72) |
+| opacity | number | No | `0.3` | Watermark opacity (0.05-1) |
+| rotation | number | No | `45` | Rotation angle in degrees (-180 to 180) |
+
+### Position Values
+
+- `tl` top-left, `tc` top-center, `tr` top-right
+- `l` center-left, `c` center, `r` center-right
+- `bl` bottom-left, `bc` bottom-center, `br` bottom-right
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/pdf/watermark-pdf \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@document.pdf" \
+  -F 'settings={"text": "CONFIDENTIAL", "position": "c", "opacity": 0.2, "rotation": 45}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/document.pdf",
+  "originalSize": 2450000,
+  "processedSize": 2500000
+}
+```
+
+## Notes
+
+- The watermark is rendered as a text overlay on each page.
+- The same watermark text, position, and style are applied uniformly to all pages.
+- Use lower opacity values (0.1-0.3) for subtle watermarks that do not obscure content.