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/database/database.go

@@ -178,7 +178,8 @@ func HashWebhookSecret(secret string) string {
 
 func (d *Database) backfillWebhookSecretHashes(ctx context.Context) error {
 	rows, err := d.database.QueryContext(ctx,
-		"SELECT id, webhook_secret FROM apps WHERE webhook_secret_hash = '' AND webhook_secret != ''")
+		"SELECT id, webhook_secret FROM apps"+
+			" WHERE webhook_secret_hash = '' AND webhook_secret != ''")
 	if err != nil {
 		return fmt.Errorf("querying apps for backfill: %w", err)
 	}