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/volume-adjust.md

@@ -0,0 +1,45 @@
+---
+description: Increase or decrease audio volume by a fixed gain in decibels.
+---
+
+# Volume Adjust
+
+Increase or decrease the volume of an audio file by applying a fixed gain in decibels.
+
+## API Endpoint
+
+`POST /api/v1/tools/audio/volume-adjust`
+
+Accepts multipart form data with an audio file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| gainDb | number | No | `3` | Volume adjustment in decibels (-30 to 30) |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/audio/volume-adjust \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@audio.mp3" \
+  -F 'settings={"gainDb": 6}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/audio.mp3",
+  "originalSize": 4500000,
+  "processedSize": 4500000
+}
+```
+
+## Notes
+
+- Positive values increase volume; negative values decrease it.
+- Large positive gains can cause clipping. Use normalize-audio for loudness-safe leveling.
+- Output format matches the input format.