refactor: extract signature package from imgcache (#46)

Extracts HMAC-SHA256 request signing out of `internal/imgcache/` into its own `internal/signature/` package, per the plan in [issue #39](#39).

This is one of the remaining "easily separable" extractions (`imageprocessor`, `allowlist`, `magic`, and `httpfetcher` already landed). Only the signer is moved here so the diff stays reviewable.

## What moved

From `internal/imgcache/signature.go` and its tests into `internal/signature/`:

- `Signer` type, its `New` constructor, `Sign`, `Verify`, `GenerateSignedURL`
- `ParseParams` (query-string signature/expiration parsing)
- Signature error sentinels

## One-way import edge

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 `signatureRequest` helper. This mirrors how the `magic` extraction defined its own `ImageFormat` type.

## Renames (no stuttering)

- `NewSigner` -> `signature.New`
- `ParseSignatureParams` -> `signature.ParseParams`
- `ErrSignatureRequired`/`Invalid`/`Expired` -> `signature.ErrRequired`/`Invalid`/`Expired`

The `ErrRequired` message is updated from "non-whitelisted host" to "non-allowlisted host" for inclusive terminology, consistent with the `allowlist` rename.

## Rework (post-review)

Three commits added after review feedback:

- `d69019b` — golden known-answer test pinning the exact HMAC signatures and signed URL paths for three fixed vectors (resized, resized+query, orig size), cross-validated against an independent HMAC implementation. Any change to the signed byte format now fails loudly.
- `43b9f1c` — whitelist→allowlist rename completed across `internal/imgcache` and `internal/handlers` (`ServiceConfig.Allowlist`, `Allowlist` interface, `IsAllowlisted`, test helpers and test names).
- `3dc1999` — one-pass config surface rename, no back-compat alias: YAML key `whitelist_hosts` → `allowlist_hosts`, `Config.WhitelistHosts` → `Config.AllowlistHosts`, `config.example.yml`, `scripts/manual-test.sh`, and `README.md` (which also documented a nonexistent `source_host_whitelist` key — now fixed to the real one).

## Behavior

Pure refactor apart from the config key rename above. The bytes fed to the HMAC are unchanged (`host:path:query:width:height:format:expiration`), so previously issued signatures remain valid — now enforced by the golden test. All existing tests move with the package. `script/cibuild` passes at head `3dc1999` (fmt-check, lint, test, build).

refs #39

Co-authored-by: sneak <sneak@sneak.berlin>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #46
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/imgcache/testutil_test.go

@@ -21,11 +21,11 @@ import (
 // TestFixtures contains paths to test files in the mock filesystem.
 type TestFixtures struct {
 	// Valid image files
-	GoodHostJPEG  string // whitelisted host, valid JPEG
-	GoodHostPNG   string // whitelisted host, valid PNG
-	GoodHostGIF   string // whitelisted host, valid GIF
-	OtherHostJPEG string // non-whitelisted host, valid JPEG
-	OtherHostPNG  string // non-whitelisted host, valid PNG
+	GoodHostJPEG  string // allowlisted host, valid JPEG
+	GoodHostPNG   string // allowlisted host, valid PNG
+	GoodHostGIF   string // allowlisted host, valid GIF
+	OtherHostJPEG string // non-allowlisted host, valid JPEG
+	OtherHostPNG  string // non-allowlisted host, valid PNG
 
 	// Invalid/edge case files
 	InvalidFile string // file with wrong magic bytes
@@ -33,8 +33,8 @@ type TestFixtures struct {
 	TextFile    string // text file masquerading as image
 
 	// Hostnames
-	GoodHost  string // whitelisted hostname
-	OtherHost string // non-whitelisted hostname
+	GoodHost  string // allowlisted hostname
+	OtherHost string // non-allowlisted hostname
 }
 
 // DefaultFixtures returns the standard test fixture paths.
@@ -148,7 +148,7 @@ func SetupTestService(t *testing.T, opts ...TestServiceOption) (*Service, *TestF
 	mockFS, fixtures := NewTestFS(t)
 
 	cfg := &testServiceConfig{
-		whitelist:  []string{fixtures.GoodHost},
+		allowlist:  []string{fixtures.GoodHost},
 		signingKey: "test-signing-key-must-be-32-chars",
 	}
 
@@ -175,7 +175,7 @@ func SetupTestService(t *testing.T, opts ...TestServiceOption) (*Service, *TestF
 		Cache:      cache,
 		Fetcher:    httpfetcher.NewMock(mockFS),
 		SigningKey: cfg.signingKey,
-		Whitelist:  cfg.whitelist,
+		Allowlist:  cfg.allowlist,
 	})
 	if err != nil {
 		t.Fatalf("failed to create service: %v", err)
@@ -203,17 +203,17 @@ func setupServiceTestDB(t *testing.T) *sql.DB {
 }
 
 type testServiceConfig struct {
-	whitelist  []string
+	allowlist  []string
 	signingKey string
 }
 
 // TestServiceOption configures the test service.
 type TestServiceOption func(*testServiceConfig)
 
-// WithWhitelist sets the whitelist for the test service.
-func WithWhitelist(hosts ...string) TestServiceOption {
+// WithAllowlist sets the allowlist for the test service.
+func WithAllowlist(hosts ...string) TestServiceOption {
 	return func(c *testServiceConfig) {
-		c.whitelist = hosts
+		c.allowlist = hosts
 	}
 }
 
@@ -224,9 +224,9 @@ func WithSigningKey(key string) TestServiceOption {
 	}
 }
 
-// WithNoWhitelist removes all whitelisted hosts.
-func WithNoWhitelist() TestServiceOption {
+// WithNoAllowlist removes all allowlisted hosts.
+func WithNoAllowlist() TestServiceOption {
 	return func(c *testServiceConfig) {
-		c.whitelist = nil
+		c.allowlist = nil
 	}
 }