Exclude image types from compression

[kevinmcconnell]

Excludes the set of common already-compressed image types, so that we don't waste time trying to gzip a compressed image.

These types are in addition to gzhttp's existing default list.

Interestingly gzhttp already excludes JPEG by default, but not the other types. We still include JPEG in our own exclusion list for clarity.

Also closes #134

[Copilot]

Pull request overview

This PR prevents gzip compression from being applied to common already-compressed image content types (e.g., PNG/WebP/AVIF/HEIC), avoiding wasted CPU/time and ensuring image responses are served as-is.

Changes:

• Add a custom gzhttp.ContentTypeFilter that builds on gzhttp.DefaultContentTypeFilter and additionally excludes common compressed image MIME types.
• Update the existing handler integration test to validate non-compression for image/png (not just JPEG, which gzhttp already excludes by default).
• Add coverage ensuring X-Sendfile-served PNG responses are not gzip-compressed, plus unit tests for the new content-type filter behavior.

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Reviewed changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated no comments.

File	Description
internal/compression_handler.go	Introduces a content-type filter to skip gzip for already-compressed image MIME types.
internal/compression_handler_test.go	Adds unit tests verifying which content types are compressed vs excluded by the new filter.
internal/handler_test.go	Updates/expands integration tests to ensure PNG (including X-Sendfile) is not gzip-compressed.

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