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/middleware/middleware.go

@@ -24,6 +24,7 @@ const CORSMaxAgeSeconds = 86400
 // Params defines dependencies for Middleware.
 type Params struct {
 	fx.In
+
 	Logger *logger.Logger
 	Config *config.Config
 }
@@ -49,6 +50,7 @@ func ipFromHostPort(hp string) string {
 	if err != nil {
 		return ""
 	}
+
 	if len(h) > 0 && h[0] == '[' {
 		return h[1 : len(h)-1]
 	}
@@ -58,6 +60,7 @@ func ipFromHostPort(hp string) string {
 
 type loggingResponseWriter struct {
 	http.ResponseWriter
+
 	statusCode   int
 	bytesWritten int64
 }
@@ -85,6 +88,7 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 			start := time.Now()
 			lrw := newLoggingResponseWriter(w)
 			ctx := r.Context()
+
 			defer func() {
 				latency := time.Since(start)
 				reqID, _ := ctx.Value(middleware.RequestIDKey).(string)