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/data/merge-csvs.md

@@ -0,0 +1,45 @@
+---
+description: Combine multiple CSV or TSV files with matching columns into one.
+---
+
+# Merge CSVs
+
+Combine multiple CSV or TSV files with matching columns into a single merged file. All input files must have the same column headers.
+
+## API Endpoint
+
+`POST /api/v1/tools/data/merge-csvs`
+
+Accepts multipart form data with two or more CSV files. No settings field is required.
+
+## Parameters
+
+This tool has no configurable parameters. Upload 2--20 CSV or TSV files with matching column headers.
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/data/merge-csvs \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@january.csv" \
+  -F "file=@february.csv" \
+  -F "file=@march.csv"
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/merged.csv",
+  "originalSize": 30000,
+  "processedSize": 28500
+}
+```
+
+## Notes
+
+- Requires between 2 and 20 input files.
+- All files must share the same column headers. The merge will fail if columns do not match.
+- The header row is included once in the output; data rows from all files are concatenated in upload order.
+- Both CSV and TSV files are accepted, but all files in a single request should use the same delimiter.