Add Favorite Snippets API with pagination

[dedukpe]

Closes #103

Summary:
Adds a Favorite Snippets API allowing users to manage their favorite code snippets.

Changes:

Database:

• Added favorites relationship table
• Added userId + snippetId indexing
• Added duplicate prevention constraint

API:

Added endpoints:

POST /api/favorites/:snippetId

• Add snippet to favorites

DELETE /api/favorites/:snippetId

• Remove snippet from favorites

GET /api/favorites

• Fetch paginated favorite snippets

Features:

• Authentication-protected routes
• Request validation
• Consistent API response format
• Pagination support
• Efficient database querying

Tests:

Added coverage for:

• Adding favorites
• Duplicate prevention
• Removing favorites
• Fetching paginated favorites
• Validation and auth errors

Documentation:

• Added frontend API usage documentation

Validation:

• Tests passing
• No existing functionality affected

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