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/pdfa-convert.md

@@ -0,0 +1,42 @@
+---
+description: Convert a PDF to archival PDF/A-2 format for long-term preservation.
+---
+
+# PDF/A Convert
+
+Convert a PDF to the PDF/A-2 archival format, suitable for long-term preservation and regulatory compliance.
+
+## API Endpoint
+
+`POST /api/v1/tools/pdf/pdfa-convert`
+
+Accepts multipart form data with a PDF file. No `settings` field is required.
+
+## Parameters
+
+This tool has no settings parameters. Upload the PDF file directly.
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/pdf/pdfa-convert \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@document.pdf"
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/document.pdf",
+  "originalSize": 2450000,
+  "processedSize": 2600000
+}
+```
+
+## Notes
+
+- The output conforms to the PDF/A-2 standard.
+- PDF/A embeds all fonts and disallows external references, so the output file may be larger than the original.
+- Encryption and JavaScript are stripped during conversion, as they are not permitted by the PDF/A standard.