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/split-csv.md

@@ -0,0 +1,47 @@
+---
+description: Split a CSV into smaller files by row count.
+---
+
+# Split CSV
+
+Split a large CSV or TSV file into smaller files by row count. Returns a ZIP archive containing the parts.
+
+## API Endpoint
+
+`POST /api/v1/tools/data/split-csv`
+
+Accepts multipart form data with a CSV file and a JSON `settings` field.
+
+## Parameters
+
+| Parameter | Type | Required | Default | Description |
+|-----------|------|----------|---------|-------------|
+| rowsPerFile | integer | No | `1000` | Number of data rows per output file (1--1,000,000) |
+| keepHeader | boolean | No | `true` | Repeat the header row in each output file |
+
+## Example Request
+
+```bash
+curl -X POST http://localhost:1349/api/v1/tools/data/split-csv \
+  -H "Authorization: Bearer si_your-api-key" \
+  -F "file=@large-dataset.csv" \
+  -F 'settings={"rowsPerFile": 500, "keepHeader": true}'
+```
+
+## Example Response
+
+```json
+{
+  "jobId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
+  "downloadUrl": "/api/v1/download/a1b2c3d4-e5f6-7890-abcd-ef1234567890/large-dataset-split.zip",
+  "originalSize": 1048576,
+  "processedSize": 1050000
+}
+```
+
+## Notes
+
+- Output is always a ZIP archive containing the split CSV parts, named sequentially (e.g. `part-1.csv`, `part-2.csv`).
+- When `keepHeader` is `true`, each part includes the original header row so each file can be used independently.
+- Both CSV and TSV files are accepted as input.
+- The row count refers to data rows only; the header row is not counted.