chore: update golangci-lint to v2.12.2 with canonical config (#54)

Canonical v2-schema `.golangci.yml`, golangci-lint pins bumped to v2.12.2 in `Dockerfile` and `script/bootstrap`, and the tree brought to `0 issues.` under it.

Three behaviour deltas: `Cache.StoreVariant` takes a context (cancelled requests skip the accounting row, recovered by reconciliation); `MetadataStorage.Store` no longer leaks `.tmp-*.json` on Write/Close/Rename failure (dead-defer bug fix); the `signing_key` too-short error text gained a `value too short:` prefix.

Eviction-loop context cancellation deferred to #102.

internal/httpfetcher/mock.go

@@ -4,12 +4,14 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"io"
 	"io/fs"
 	"net/http"
 	"strings"
 )
 
+// errEmptyURLPath is returned when a mock URL has no usable path.
+var errEmptyURLPath = errors.New("empty URL path")
+
 // MockFetcher implements Fetcher using an embedded filesystem.
 // Files are organized as: hostname/path/to/file.ext
 // URLs like https://example.com/images/photo.jpg map to example.com/images/photo.jpg.
@@ -59,7 +61,7 @@ func (m *MockFetcher) Fetch(ctx context.Context, url string) (*FetchResult, erro
 	contentType := detectContentTypeFromPath(path)
 
 	return &FetchResult{
-		Content:       f.(io.ReadCloser),
+		Content:       f,
 		ContentLength: stat.Size(),
 		ContentType:   contentType,
 		Headers:       make(http.Header),
@@ -86,7 +88,7 @@ func urlToFSPath(rawURL string) (string, error) {
 	}
 
 	if url == "" {
-		return "", errors.New("empty URL path")
+		return "", errEmptyURLPath
 	}
 
 	return url, nil
@@ -98,18 +100,18 @@ func detectContentTypeFromPath(path string) string {
 
 	switch {
 	case strings.HasSuffix(path, ".jpg"), strings.HasSuffix(path, ".jpeg"):
-		return "image/jpeg"
+		return contentTypeJPEG
 	case strings.HasSuffix(path, ".png"):
-		return "image/png"
+		return contentTypePNG
 	case strings.HasSuffix(path, ".gif"):
-		return "image/gif"
+		return contentTypeGIF
 	case strings.HasSuffix(path, ".webp"):
-		return "image/webp"
+		return contentTypeWebP
 	case strings.HasSuffix(path, ".avif"):
-		return "image/avif"
+		return contentTypeAVIF
 	case strings.HasSuffix(path, ".svg"):
-		return "image/svg+xml"
+		return contentTypeSVG
 	default:
-		return "application/octet-stream"
+		return contentTypeOctetStream
 	}
 }