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/session/session.go

@@ -107,7 +107,9 @@ func (m *Manager) ValidateSession(r *http.Request) (*Data, error) {
 	}
 
 	var data Data
-	if err := m.sc.Decode(CookieName, cookie.Value, &data); err != nil {
+
+	err = m.sc.Decode(CookieName, cookie.Value, &data)
+	if err != nil {
 		return nil, ErrInvalidSession
 	}