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/flatten-pdf.md

@@ -0,0 +1,43 @@
+---
+description: Bake forms and annotations into page content.
+---
+
+# Flatten PDF
+
+Bake interactive form fields and annotations into the page content, producing a static PDF that looks the same everywhere.
+
+## API Endpoint
+
+`POST /api/v1/tools/pdf/flatten-pdf`
+
+Accepts multipart form data with a PDF file.
+
+## Parameters
+
+This tool has no configurable parameters. Upload a PDF and all forms and annotations will be flattened.
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/pdf/flatten-pdf \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@form.pdf"
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/form.pdf",
+  "originalSize": 185000,
+  "processedSize": 172000
+}
+```
+
+## Notes
+
+- Accepted input format: `.pdf`.
+- This is a fast (synchronous) tool that returns the result directly.
+- Form field values are preserved as static text in the output.
+- Annotations (comments, highlights, sticky notes) become part of the page content and can no longer be edited.