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/merge-videos.md

@@ -0,0 +1,43 @@
+---
+description: Join multiple video clips into one file.
+---
+
+# Merge Videos
+
+Join multiple video clips into a single MP4 file. All inputs are normalized to the first video's resolution and 30 fps.
+
+## API Endpoint
+
+`POST /api/v1/tools/video/merge-videos`
+
+Accepts multipart form data with two or more video files. This is an async endpoint - it returns `202 Accepted` immediately and progress is streamed via SSE at `GET /api/v1/jobs/{jobId}/progress`.
+
+## Parameters
+
+This tool has no settings parameters. Upload 2-10 video files as multiple `file` parts.
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/video/merge-videos \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@intro.mp4" \
+  -F "file=@main.mp4" \
+  -F "file=@outro.mp4"
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "async": true
+}
+```
+
+## Notes
+
+- Clips are concatenated in the order they are uploaded.
+- All clips are re-encoded to match the first clip's resolution, frame rate (30 fps), and codec (H.264). Mismatched inputs are automatically normalized.
+- Accepts 2-10 video files per request.
+- Progress updates are available via SSE at `GET /api/v1/jobs/{jobId}/progress` until the job completes.