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/markdown-to-pdf.md

@@ -0,0 +1,43 @@
+---
+description: Convert a Markdown file to a styled PDF.
+---
+
+# Markdown to PDF
+
+Convert a Markdown file to a styled PDF document. Remote resources are disabled for privacy.
+
+## API Endpoint
+
+`POST /api/v1/tools/pdf/markdown-to-pdf`
+
+Accepts multipart form data with a Markdown file.
+
+## Parameters
+
+This tool has no configurable parameters. Upload a Markdown file and it will be converted to PDF.
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/pdf/markdown-to-pdf \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@document.md"
+```
+
+## 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 formats: `.md`, `.markdown`.
+- Remote resources (images, stylesheets referenced via URLs) are not fetched for privacy and security.
+- The Markdown is first rendered to HTML, then converted to PDF via WeasyPrint.
+- Code blocks, tables, and other Markdown elements are styled in the PDF output.