Import snippets from JSON

[byteofhoney]

Goal:

Let users upload a JSON file to bulk add snippets back into the application.

User story:

As a user, I want to import snippets from a JSON file so that I can restore or migrate my stash.

Tasks

• Add a new route, e.g. GET/POST /import, with a file upload form
• Validate uploaded JSON matches expected snippet structure before inserting
• Decide and document behavior for duplicates (skip / overwrite / insert as new)
• Show a summary flash message after import (e.g. "12 snippets imported")
• Add a visible "Import" button or link somewhere in the UI

Acceptance Criteria

• Uploading a valid exported JSON file successfully re-creates those snippets
• Invalid/malformed JSON shows a clear error instead of crashing
• Duplicate handling behaves as documented in the PR