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/render_template_test.go

@@ -16,7 +16,9 @@ func TestRenderTemplateBuffersOutput(t *testing.T) {
 	testCtx := setupTestHandlers(t)
 
 	// The setup page is simple and has no DB dependencies
-	request := httptest.NewRequest(http.MethodGet, "/setup", nil)
+	request := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/setup", nil,
+	)
 	recorder := httptest.NewRecorder()
 
 	handler := testCtx.handlers.HandleSetupGET()
@@ -39,7 +41,7 @@ func TestDashboardRenderTemplateBuffersOutput(t *testing.T) {
 
 	testCtx := setupTestHandlers(t)
 
-	request := httptest.NewRequest(http.MethodGet, "/", nil)
+	request := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", nil)
 	recorder := httptest.NewRecorder()
 
 	handler := testCtx.handlers.HandleDashboard()
@@ -59,7 +61,9 @@ func TestLoginRenderTemplateBuffersOutput(t *testing.T) {
 
 	testCtx := setupTestHandlers(t)
 
-	request := httptest.NewRequest(http.MethodGet, "/login", nil)
+	request := httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/login", nil,
+	)
 	recorder := httptest.NewRecorder()
 
 	handler := testCtx.handlers.HandleLoginGET()