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/pdf-metadata.md

@@ -0,0 +1,58 @@
+---
+description: Read and write PDF document metadata.
+---
+
+# PDF Metadata
+
+Read and update PDF document metadata fields such as title, author, subject, and keywords. When no settings are provided, the existing metadata is returned without modification.
+
+## API Endpoint
+
+`POST /api/v1/tools/pdf/pdf-metadata`
+
+Accepts multipart form data with a PDF file and an optional JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| title | string | No | - | Document title (max 500 characters) |
+| author | string | No | - | Document author (max 500 characters) |
+| subject | string | No | - | Document subject (max 500 characters) |
+| keywords | string | No | - | Document keywords (max 500 characters) |
+
+All parameters are optional. Omitted fields are left unchanged.
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/pdf/pdf-metadata \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@report.pdf" \
+  -F 'settings={"title": "Q2 Report", "author": "Finance Team"}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/report.pdf",
+  "originalSize": 245000,
+  "processedSize": 245200,
+  "metadata": {
+    "title": "Q2 Report",
+    "author": "Finance Team",
+    "subject": "",
+    "keywords": ""
+  }
+}
+```
+
+## Notes
+
+- Accepted input format: `.pdf`.
+- This is a fast (synchronous) tool that returns the result directly.
+- The `metadata` field in the response contains the resulting metadata after any updates.
+- To read metadata without modifying it, omit the `settings` field or send an empty object.
+- Each metadata field is limited to 500 characters.