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/pdf/epub-convert.md

@@ -0,0 +1,46 @@
+---
+description: Convert an EPUB to PDF, DOCX, HTML, or Markdown.
+---
+
+# Convert EPUB
+
+Convert an EPUB e-book to PDF, Word (DOCX), HTML, or Markdown. Remote resources inside the book are not fetched.
+
+## API Endpoint
+
+`POST /api/v1/tools/pdf/epub-convert`
+
+Accepts multipart form data with an EPUB file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| format | string | Yes | - | Output format: `pdf`, `docx`, `html`, `md` |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/pdf/epub-convert \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@book.epub" \
+  -F 'settings={"format": "pdf"}'
+```
+
+## Example Response
+
+Returns `202 Accepted`. Track progress via SSE at `/api/v1/jobs/{jobId}/progress`.
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "async": true
+}
+```
+
+## Notes
+
+- Accepted input format: `.epub`.
+- Remote resources embedded in the EPUB (external images, fonts) are not fetched for security.
+- Image fidelity in the converted output may vary depending on the EPUB structure.
+- Conversion is handled by Pandoc on the server.