Update golangci-lint to v2.12.2 with canonical config

Bump golangci-lint from v2.10.1 to v2.12.2 in the Dockerfile lint
stage (tag+digest pin) and script/bootstrap release-archive pins
(linux amd64/arm64 sha256s). Replace .golangci.yml with the canonical
v2-layout config so linter settings (lll 88, funlen 80/50, cyclop 15,
dupl 100) actually apply.

Fix all findings surfaced by the new linter and config:

- noctx: use httptest.NewRequestWithContext in all tests
- gosec G710/G703: route app redirects through a path-escaping
  redirectToApp helper; annotate internal log path usage
- goconst: introduce shared constants for template/JSON keys and
  repeated test literals
- lll: wrap lines to the 88-column limit
- dupl: extract shared helpers (generic findAllByAppID in models,
  deleteAppResource in handlers, parsePush in webhook payloads,
  table-driven/helper-based test dedup)
- nolintlint: drop nolint directives made obsolete by the new limits

Record the change in TODO.md; make check is green.

internal/handlers/handlers.go

@@ -22,6 +22,19 @@ import (
 	"sneak.berlin/go/upaas/templates"
 )
 
+// Template data keys shared across handlers.
+const (
+	dataKeyApp   = "App"
+	dataKeyError = "Error"
+)
+
+// JSON response keys shared across handlers.
+const (
+	jsonKeyError  = "error"
+	jsonKeyLogs   = "logs"
+	jsonKeyStatus = "status"
+)
+
 // Params contains dependencies for Handlers.
 type Params struct {
 	fx.In