feat(i18n): 21-language pipeline, landing/docs/API wiring, landing+API translations

Shared Claude Code translation pipeline (scripts/i18n, no API key) plus Astro/VitePress/Scalar i18n wiring. Landing and API reference translated into all 20 languages; docs i18n wiring + English source anchors. The translated docs markdown (apps/docs/<locale>/**, 3,620 files) follows in a companion PR because it exceeds GitHub's per-PR CI file limit.

apps/docs/tools/video/convert-video.md

@@ -2,24 +2,24 @@
 description: Convert videos between MP4, MOV, WebM, AVI, and MKV.
 ---
 
-# Convert Video
+# Convert Video {#convert-video}
 
 Convert videos between MP4, MOV, WebM, AVI, and MKV formats with configurable quality presets.
 
-## API Endpoint
+## API Endpoint {#api-endpoint}
 
 `POST /api/v1/tools/video/convert-video`
 
 Accepts multipart form data with a video file and a JSON `settings` field. This is an async endpoint - it returns `202 Accepted` immediately and progress is streamed via SSE at `GET /api/v1/jobs/{jobId}/progress`.
 
-## Parameters
+## Parameters {#parameters}
 
 | Parameter | Type | Required | Default | Description |
 |-----------|------|----------|---------|-------------|
 | format | string | No | `"mp4"` | Output format: `mp4`, `mov`, `webm`, `avi`, `mkv` |
 | quality | string | No | `"balanced"` | Quality preset: `high`, `balanced`, `small` |
 
-## Example Request
+## Example Request {#example-request}
 
 ```bash
 curl -X POST http://localhost:1349/api/v1/tools/video/convert-video \
@@ -28,7 +28,7 @@ curl -X POST http://localhost:1349/api/v1/tools/video/convert-video \
   -F 'settings={"format": "webm", "quality": "balanced"}'
 ```
 
-## Example Response
+## Example Response {#example-response}
 
 ```json
 {
@@ -37,7 +37,7 @@ curl -X POST http://localhost:1349/api/v1/tools/video/convert-video \
 }
 ```
 
-## Notes
+## Notes {#notes}
 
 - The `high` quality preset produces the best visual fidelity but larger files. The `small` preset aggressively compresses for minimum file size.
 - WebM output uses VP9 encoding. MP4 and MOV use H.264. AVI and MKV are available for legacy or archival workflows.