Fix: Implement user-specific snippet filtering

[hexawulf]

Resolves data isolation issue where you could see snippets from other accounts.

Changes:

• Modified DatabaseStorage.getSnippets in server/storage.ts to accept an optional userId filter and apply it to the database query.
• Modified SimpleStorage.getSnippets in server/simple-storage.ts to accept an optional userId filter and apply it to the database query.
• Applied authMiddleware to the GET /api/snippets endpoint in server/routes.ts to enforce authentication.
• The GET /api/snippets endpoint now extracts the authenticated userId and passes it to the storage methods (storage.getSnippets and simpleStorage.getSnippets) to ensure you only retrieve your own snippets.
• Updated the fallback direct database query in GET /api/snippets to also filter by userId.

Testing:

• Added new API tests in server/__tests__/routes.test.ts with multiple mock users.
• Tests verify that:
  • Authenticated users only retrieve their own snippets via GET /api/snippets.
  • Unauthenticated requests to GET /api/snippets are rejected (401).
  • Users with no snippets receive an empty array.
• Existing tests for public snippet access (/api/public/snippets) were preserved and confirmed to be working correctly.