Move HMAC-SHA256 request signing out of internal/imgcache/ into its own
internal/signature/ package, per the plan in issue #39. This is one of the
remaining "easily separable" extractions; only the signer is moved here so
the diff stays reviewable.
What moved (from internal/imgcache/signature.go):
- Signer type, its New constructor, Sign, Verify, GenerateSignedURL
- ParseParams (query-string signature/expiration parsing)
- Signature error sentinels
To keep the import edge one-way (imgcache depends on signature, never the
reverse), the package defines a standalone Request type carrying just the
fields the signature covers, instead of importing imgcache.ImageRequest.
imgcache projects its ImageRequest onto signature.Request via a small
unexported helper.
Renames (no stuttering):
- NewSigner -> signature.New
- ParseSignatureParams -> signature.ParseParams
- ErrSignatureRequired/Invalid/Expired -> signature.ErrRequired/Invalid/Expired
The "non-whitelisted host" error message is updated to "non-allowlisted"
for inclusive terminology, consistent with the allowlist rename.
Pure refactor. The bytes fed to the HMAC are unchanged, so signatures remain
compatible. All existing tests move with the package; docker build passes
(fmt-check, lint, test, build).
refs #39
