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/imgcache/urlparser.go

@@ -40,7 +40,8 @@ type ParsedURL struct {
 	Format ImageFormat
 }
 
-// ParseImagePath parses the path captured by chi's wildcard: <host>/<path>/<size>.<format>
+// ParseImagePath parses the path captured by chi's wildcard:
+// <host>/<path>/<size>.<format>
 // This is the primary entry point when using chi routing.
 // Examples:
 //   - cdn.example.com/photos/cat.jpg/800x600.webp
@@ -76,7 +77,8 @@ func ParseImageURL(urlPath string) (*ParsedURL, error) {
 // parseImageComponents parses <host>/<path>/<size>.<format> structure.
 func parseImageComponents(remainder string) (*ParsedURL, error) {
 	// Check for path traversal before any other processing
-	if err := checkPathTraversal(remainder); err != nil {
+	err := checkPathTraversal(remainder)
+	if err != nil {
 		return nil, err
 	}
 
@@ -102,6 +104,7 @@ func parseImageComponents(remainder string) (*ParsedURL, error) {
 	// Split host from path
 	// The first segment is the host, everything after is the path
 	firstSlash := strings.Index(hostAndPath, "/")
+
 	var host, path, query string
 
 	if firstSlash == -1 {
@@ -181,8 +184,7 @@ func checkPathTraversal(path string) error {
 
 	// Also check for ".." as a path segment in the original path
 	// This catches cases where the path hasn't been normalized
-	segments := strings.Split(path, "/")
-	for _, seg := range segments {
+	for seg := range strings.SplitSeq(path, "/") {
 		// URL decode the segment
 		decodedSeg, _ := url.PathUnescape(seg)
 		decodedSeg = strings.ReplaceAll(decodedSeg, "\\", "/")
@@ -202,8 +204,10 @@ func parseSizeFormat(s string) (Size, ImageFormat, error) {
 		return Size{}, "", ErrInvalidSize
 	}
 
-	var size Size
-	var formatStr string
+	var (
+		size      Size
+		formatStr string
+	)
 
 	if matches[4] == "orig" {
 		// "orig.format" pattern