Export snippets to JSON

[byteofhoney]

Goal: Let users download all their snippets as a single JSON file for backup.
User story: As a user, I want to export my snippets to JSON so that I always have a backup outside the application.

Steps and Rules

• Add a new route, e.g. GET /export
• Query all snippets from the database
• Convert to JSON, handle the _id field (convert ObjectId to string)
• Return as a downloadable .json file (correct Content-Disposition header)
• Add a visible "Export" button in the UI

Acceptance Criteria

• Clicking Export downloads a valid .json file
• File contains every snippet currently in the database
• Downloaded file can be opened and read without errors