Define unified /v1/chat/completions endpoint and stub routing logic

[gorkem-bwl]

Lay out the “front door” of our proxy API. Create the Express route handler for /v1/chat/completions and wire it to a placeholder routing engine.

• New route in routes/chat.ts or similar
• Handler calls routingEngine.routeRequest() with the raw payload
• Returns a 501 “Not implemented” for now
• Unit test verifies endpoint exists and returns 501