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>

README.md

@@ -29,7 +29,7 @@ Image-heavy web applications need a fast, caching reverse proxy that
 can resize and transcode images on the fly. pixa fills that role as a
 single, self-contained binary with no external runtime dependencies
 beyond libvips. It supports HMAC-SHA256 signed URLs with expiration to
-prevent abuse, and whitelisted source hosts for open access.
+prevent abuse, and allowlisted source hosts for open access.
 
 ## Design
 
@@ -61,7 +61,7 @@ Images are only fetched from origins using TLS with valid certificates.
 
 ### Source Hosts
 
-Source hosts may be whitelisted in the configuration. Non-whitelisted
+Source hosts may be allowlisted in the configuration. Non-allowlisted
 hosts require an HMAC-SHA256 signature.
 
 #### Signature Specification
@@ -99,7 +99,7 @@ expiration 1704067200:
 4. URL:
    `/v1/image/cdn.example.com/photos/cat.jpg/800x600.webp?sig=<base64url>&exp=1704067200`
 
-**Whitelist patterns:**
+**Allowlist patterns:**
 
 - **Exact match**: `cdn.example.com` — matches only that host
 - **Suffix match**: `.example.com` — matches `cdn.example.com`,
@@ -110,7 +110,7 @@ expiration 1704067200:
 Configured via YAML file (`--config`). Key settings:
 
 - `access_control_allow_origin` — CORS origin
-- `source_host_whitelist` — list of allowed upstream hosts
+- `allowlist_hosts` — list of allowed upstream hosts
 - `upstream_fetch_timeout` — timeout for origin requests
 - `upstream_max_response_size` — max origin response size
 - `downstream_timeout` — client response timeout