fix(mcp): accept x-api-key as an alternative authentication header

[anassg-lago]

Claude's connector setup rejects the custom x-lago-api-key header. Accept x-api-key as an alternative in both shared MCP API configuration paths so clients can use a standard header name.

Existing clients keep working: x-lago-api-key takes precedence when both headers are supplied, and environment credential fallback and authentication to Lago API are unchanged.

Validation:

• 22 HTTP integration scenarios against a mock Lago API cover SDK-backed customer calls and direct HTTP event calls, both header names, case insensitivity, precedence, missing and invalid keys, and environment fallback. The new-header scenario failed before the change and passes after it.
• Formatting, Clippy with warnings denied, default and all-feature test suites, release build, and Cargo publish dry run passed. The dry run reported existing package metadata and yanked dependency warnings.

Actual Claude-to-Lago access still needs validation after deploying the modified image.