feat: stabilize rate limiting behaviour

[gildesmarais]

Addresses feedback from PR #1011:

• Retry-After uses ||= for 503/504
• Duplicate CODE strings removed from resolve_error_code
• Rate limiter overflow eviction avoids materializing all keys via streaming iteration
• Added regression tests for Retry-After preservation

[Copilot]

Pull request overview

This PR refines and stabilizes rate-limiting and error-response behavior in html2rss-web, focusing on preserving caller-provided Retry-After headers for timeout-related errors, reducing duplicated error-code strings, and improving overflow eviction to avoid materializing all rate-limiter keys.

Changes:

• Preserve an existing Retry-After header for 503/504 responses by switching to ||= assignment.
• Simplify resolve_error_code by relying on error CODE constants / #code where appropriate.
• Adjust rate limiter overflow eviction to iterate keys rather than materializing all keys, and add regression specs for Retry-After preservation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
spec/html2rss/web/error_responder_spec.rb	Adds regression coverage ensuring existing Retry-After headers are preserved for 503/504.
app/web/request/rate_limiter.rb	Changes overflow eviction strategy to avoid allocating @history.keys during eviction.
app/web/errors/error_responder.rb	Preserves pre-set Retry-After for 503/504 and deduplicates error code resolution via constants.