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/image/blur-background.md

@@ -0,0 +1,48 @@
+---
+description: Blur the background while keeping the subject sharp using AI.
+---
+
+# Blur Background
+
+Blur the background of an image while keeping the subject sharp. The AI model isolates the subject, applies a blur to the original background, and composites the sharp subject on top.
+
+## API Endpoint
+
+`POST /api/v1/tools/image/blur-background`
+
+Accepts multipart form data with an image file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| intensity | integer | No | `50` | Blur intensity (1-100) |
+| feather | integer | No | `0` | Edge feathering radius (0-20) |
+| format | string | No | `"png"` | Output format: `png` or `webp` |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/image/blur-background \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@photo.jpg" \
+  -F 'settings={"intensity": 75, "feather": 3}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "async": true
+}
+```
+
+Track progress via SSE at `GET /api/v1/jobs/{jobId}/progress`. When the job completes, the SSE stream emits a `completed` event with the download URL.
+
+## Notes
+
+- This is an AI-powered tool that returns `202 Accepted` and processes asynchronously. Connect to the SSE endpoint to receive progress updates and the final result.
+- Requires the **background-removal** feature bundle to be installed. Returns `501` if the bundle is not available.
+- Higher intensity values produce a stronger blur effect. Values above 80 create a pronounced bokeh-like separation.
+- HEIC, RAW, PSD, and SVG inputs are automatically decoded before processing.