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/healthcheck/healthcheck.go

@@ -16,6 +16,7 @@ import (
 // Params defines dependencies for Healthcheck.
 type Params struct {
 	fx.In
+
 	Globals  *globals.Globals
 	Config   *config.Config
 	Logger   *logger.Logger
@@ -53,6 +54,8 @@ func New(lc fx.Lifecycle, params Params) (*Healthcheck, error) {
 }
 
 // Response is the JSON response for health checks.
+//
+//nolint:tagliatelle // health endpoint response format uses snake_case
 type Response struct {
 	Status        string `json:"status"`
 	Now           string `json:"now"`
@@ -63,10 +66,6 @@ type Response struct {
 	Maintenance   bool   `json:"maintenance_mode"`
 }
 
-func (s *Healthcheck) uptime() time.Duration {
-	return time.Since(s.StartupTime)
-}
-
 // Healthcheck returns the current health status.
 func (s *Healthcheck) Healthcheck() *Response {
 	resp := &Response{
@@ -81,3 +80,7 @@ func (s *Healthcheck) Healthcheck() *Response {
 
 	return resp
 }
+
+func (s *Healthcheck) uptime() time.Duration {
+	return time.Since(s.StartupTime)
+}