docs: add example curl commands to API reference

[TrueFurina]

Summary

Per #47: docs/API.md described every endpoint but had no example invocations, so first-time developers couldn't quickly verify the API works.

Changes

• docs/API.md: added example curl commands for every endpoint:
  • GET /health
  • POST /auth/register, POST /auth/login (with example JWT response note)
  • POST /profiles, GET /profiles/{id}, DELETE /profiles/{id}
  • POST /reviews, GET /reviews/{id}, GET /reviews (with pagination query example)
• Authenticated endpoints show Authorization: Bearer <jwt>; example request bodies use the documented payload shapes

Acceptance criteria

• Every endpoint in API.md has an example invocation
• Examples use the real endpoint paths and payloads from the codebase