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/audio/merge-audio.md

@@ -0,0 +1,48 @@
+---
+description: Combine multiple audio files into one sequential track.
+---
+
+# Merge Audio
+
+Combine two or more audio files into a single sequential track, concatenated in the order they are uploaded.
+
+## API Endpoint
+
+`POST /api/v1/tools/audio/merge-audio`
+
+Accepts multipart form data with multiple audio files and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| format | string | No | `"mp3"` | Output format: `mp3`, `wav`, `flac`, `m4a` |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/audio/merge-audio \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@intro.mp3" \
+  -F "file=@main.mp3" \
+  -F "file=@outro.mp3" \
+  -F 'settings={"format": "mp3"}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/merged.mp3",
+  "originalSize": 9500000,
+  "processedSize": 9200000
+}
+```
+
+## Notes
+
+- Accepts 2 to 10 audio files per request.
+- Files are concatenated in upload order.
+- All input files are re-encoded to the chosen output format and sample rate for seamless joining.
+- Mixed input formats are supported (e.g. one WAV and one MP3).