Added Chi Routing, API Stubs

[Howard-nolan]

Change Summary:

• Added Chi router setup to handle API routes.
• Created stub handlers for /v1/r/{code} and /v1/shorten endpoints to prepare for future URL redirection and shortening logic.
• This sets the foundation for versioned API routing and future OpenAPI documentation integration.

How It Works:

• The server now initializes a Chi router in main.go, registering routes under /v1/.
• GET /v1/r/{code} will later handle redirecting a short code to its target URL.
• POST /v1/shorten will accept a long URL in the request body and return a shortened code.
• Both endpoints currently return placeholder JSON responses to confirm routing is functional.

Additional Notes (optional):

• Future work: implement persistence layer for shortened URLs (e.g., MongoDB, Redis).
• Can easily extend versioning by adding additional route groups (e.g., /v2/).
• No functional logic yet—this MR only sets up routing and scaffolding.