feat(docs): add /api/revalidate endpoint with default all-locale revalidation

[dsinghvi]

Description

This PR adds a new /api/revalidate endpoint to the docs preview server that revalidates locale translations on demand.

Changes Made

• Added /api/revalidate POST endpoint to both Express (Node.js) and Bun server implementations
• Default behavior: Revalidates all configured locales when no locale parameter is provided
• Specific locale: Revalidates only the specified locale when locale parameter is passed (e.g., {"locale": "fr"})
• Returns a JSON response with the list of revalidated locale codes
• Includes proper error handling for missing docs definitions and invalid locale codes

API Usage

Revalidate all locales (default):

curl -X POST http://localhost:PORT/api/revalidate -H "Content-Type: application/json" -d '{}'
# Response: {"revalidated": ["fr", "es", "de"]}

Revalidate specific locale:

curl -X POST http://localhost:PORT/api/revalidate -H "Content-Type: application/json" -d '{"locale": "fr"}'
# Response: {"revalidated": ["fr"]}

Testing

• Code compiles successfully
• Formatting and linting checks pass
• Manual testing with docs preview server (requires running server with translated content)
• Unit tests (follow-up if needed)

Slack Thread