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/replace-audio.md

@@ -0,0 +1,43 @@
+---
+description: Swap the audio track of a video with another file.
+---
+
+# Replace Audio
+
+Swap the audio track of a video with an audio file. Upload both a video and an audio file.
+
+## API Endpoint
+
+`POST /api/v1/tools/video/replace-audio`
+
+Accepts multipart form data with exactly two files: a video file followed by an audio file.
+
+## Parameters
+
+This tool has no settings parameters. Upload a video file and an audio file as two `file` parts.
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/video/replace-audio \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@clip.mp4" \
+  -F "file=@voiceover.mp3"
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/clip.mp4",
+  "originalSize": 12500000,
+  "processedSize": 13100000
+}
+```
+
+## Notes
+
+- Exactly two files must be uploaded: the first must be a video, the second must be an audio file.
+- If the audio file is longer than the video, it is trimmed to match the video duration. If shorter, the remaining video plays in silence.
+- The video stream is copied without re-encoding, so there is no video quality loss.