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/imgcache.go

@@ -90,6 +90,7 @@ func (r *ImageRequest) SourceURL() string {
 	if r.AllowHTTP {
 		scheme = "http"
 	}
+
 	url := scheme + "://" + r.SourceHost + r.SourcePath
 	if r.SourceQuery != "" {
 		url += "?" + r.SourceQuery