feat(http): implement HTTP timeout handling for fetch requests

[esgn]

Closes #45

This PR adds timeout handling to the shared HTTP helpers used for outbound calls to IGN/GPF services.

Without a timeout, MCP tool calls could hang indefinitely when an upstream service is slow or unresponsive. This change makes those failures bounded and explicit.

What changed

• Wrap fetchJSONGet() and fetchJSONPost() with timeout handling in src/helpers/http.ts
• Abort stalled outbound requests with AbortController
• Convert timeout aborts into structured ServiceResponseErrors with:
  • HTTP status 504
  • status text Gateway Timeout
  • service code TIMEOUT
• Add HTTP_TIMEOUT configuration in seconds
• Document the new setting in README.md
• Add test coverage for GET and POST timeout behavior in test/helpers/http.test.ts

Configuration

• New env var: HTTP_TIMEOUT
• Unit: seconds
• Default: 15

Example:

export HTTP_TIMEOUT=15

Why
This prevents the MCP server from waiting forever on upstream HTTP calls and gives clients a predictable error instead of a hanging tool invocation.

Tests

• npm test -- test/helpers/http.test.ts
• npm run typecheck