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/change-fps.md

@@ -0,0 +1,45 @@
+---
+description: Change the frame rate of a video.
+---
+
+# Change FPS
+
+Change the frame rate of a video to a target value between 1 and 120 fps.
+
+## API Endpoint
+
+`POST /api/v1/tools/video/change-fps`
+
+Accepts multipart form data with a video file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| fps | number | No | `30` | Target frame rate (1-120) |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/video/change-fps \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@clip.mp4" \
+  -F 'settings={"fps": 24}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/clip.mp4",
+  "originalSize": 12500000,
+  "processedSize": 10200000
+}
+```
+
+## Notes
+
+- Lowering the frame rate drops frames and reduces file size. Increasing it duplicates frames to fill the gap but does not add real motion detail.
+- Common target values: 24 (cinema), 30 (web/broadcast), 60 (smooth playback).
+- The audio track is preserved at its original sample rate.