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.

cmd/pixad/main.go

@@ -30,7 +30,8 @@ func main() {
 
 	rootCmd.Flags().StringVarP(&configPath, "config", "c", "", "path to config file")
 
-	if err := rootCmd.Execute(); err != nil {
+	err := rootCmd.Execute()
+	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}