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/config/config.go

@@ -41,7 +41,7 @@ type Config struct {
 
 	// Image proxy settings
 	SigningKey                 string   // HMAC signing key for URL signatures
-	WhitelistHosts             []string // Hosts that don't require signatures
+	AllowlistHosts             []string // Hosts that don't require signatures
 	AllowHTTP                  bool     // Allow non-TLS upstream (testing only)
 	UpstreamConnectionsPerHost int      // Max concurrent connections per upstream host
 }
@@ -69,7 +69,7 @@ func New(_ fx.Lifecycle, params Params) (*Config, error) {
 		MetricsUsername:            getString(sc, "metrics.username", ""),
 		MetricsPassword:            getString(sc, "metrics.password", ""),
 		SigningKey:                 getString(sc, "signing_key", ""),
-		WhitelistHosts:             getStringSlice(sc, "whitelist_hosts"),
+		AllowlistHosts:             getStringSlice(sc, "allowlist_hosts"),
 		AllowHTTP:                  getBool(sc, "allow_http", false),
 		UpstreamConnectionsPerHost: getInt(sc, "upstream_connections_per_host", DefaultUpstreamConnectionsPerHost),
 	}