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/reverse-video.md

@@ -2,21 +2,21 @@
 description: Play a video clip backwards.
 ---
 
-# Reverse Video
+# Reverse Video {#reverse-video}
 
 Play a video clip backwards. The audio track is also reversed.
 
-## API Endpoint
+## API Endpoint {#api-endpoint}
 
 `POST /api/v1/tools/video/reverse-video`
 
 Accepts multipart form data with a video file. This tool has no configurable settings.
 
-## Parameters
+## Parameters {#parameters}
 
 This tool has no parameters. It reverses the entire video.
 
-## Example Request
+## Example Request {#example-request}
 
 ```bash
 curl -X POST http://localhost:1349/api/v1/tools/video/reverse-video \
@@ -24,7 +24,7 @@ curl -X POST http://localhost:1349/api/v1/tools/video/reverse-video \
   -F "file=@clip.mp4"
 ```
 
-## Example Response
+## Example Response {#example-response}
 
 ```json
 {
@@ -35,7 +35,7 @@ curl -X POST http://localhost:1349/api/v1/tools/video/reverse-video \
 }
 ```
 
-## Notes
+## Notes {#notes}
 
 - Limited to clips up to 5 minutes in length. Longer videos are rejected with a 400 error.
 - Both video and audio tracks are reversed. To reverse video without audio, mute it first.