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/video/watermark-video.md

@@ -0,0 +1,55 @@
+---
+description: Burn a text watermark onto video frames.
+---
+
+# Watermark Video
+
+Burn a text watermark onto every frame of a video with configurable position, size, opacity, and color.
+
+## API Endpoint
+
+`POST /api/v1/tools/video/watermark-video`
+
+Accepts multipart form data with a video file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| text | string | Yes | - | Watermark text (1-200 characters) |
+| position | string | No | `"br"` | Position on the frame: `tl`, `tc`, `tr`, `l`, `c`, `r`, `bl`, `bc`, `br` |
+| fontSize | integer | No | `36` | Font size in pixels (8-120) |
+| opacity | number | No | `0.5` | Watermark opacity (0.05-1) |
+| color | string | No | `"#ffffff"` | Hex color for the text (e.g. `"#ffffff"`) |
+
+### Position Values
+
+- **tl** - Top left, **tc** - Top center, **tr** - Top right
+- **l** - Middle left, **c** - Center, **r** - Middle right
+- **bl** - Bottom left, **bc** - Bottom center, **br** - Bottom right
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/video/watermark-video \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@clip.mp4" \
+  -F 'settings={"text": "PREVIEW", "position": "c", "fontSize": 48, "opacity": 0.3}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/clip.mp4",
+  "originalSize": 12500000,
+  "processedSize": 12400000
+}
+```
+
+## Notes
+
+- The watermark is permanently rendered into the video frames and cannot be removed after processing.
+- The watermark uses a sans-serif font built into FFmpeg.
+- For image watermarks, use the image Watermark tool instead.