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/allowlist/allowlist.go

@@ -10,7 +10,8 @@ import (
 type HostAllowList struct {
 	// exactHosts contains hosts that must match exactly (e.g., "cdn.example.com")
 	exactHosts map[string]struct{}
-	// suffixHosts contains domain suffixes to match (e.g., ".example.com" matches "cdn.example.com")
+	// suffixHosts contains domain suffixes to match
+	// (e.g., ".example.com" matches "cdn.example.com")
 	suffixHosts []string
 }