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.

script/bootstrap

@@ -10,11 +10,11 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-07-07. Never "latest"; exact versions only.
-GOLANGCI_LINT_VERSION="2.10.1"
-# sha256 of golangci-lint-2.10.1-linux-<arch>.tar.gz release archives
-GOLANGCI_LINT_SHA256_AMD64="dfa775874cf0561b404a02a8f4481fc69b28091da95aa697259820d429b09c99"
-GOLANGCI_LINT_SHA256_ARM64="6652b42ae02915eb2f9cb2a2e0cac99514c8eded8388d88ae3e06e1a52c00de8"
+# Pinned versions, 2026-08-07. Never "latest"; exact versions only.
+GOLANGCI_LINT_VERSION="2.12.2"
+# sha256 of golangci-lint-2.12.2-linux-<arch>.tar.gz release archives
+GOLANGCI_LINT_SHA256_AMD64="8df580d2670fed8fa984aac0507099af8df275e665215f5c7a2ae3943893a553"
+GOLANGCI_LINT_SHA256_ARM64="44cd40a8c76c86755375adfeea52cfd3533cb43d7bd647771e0ae065e166df3a"
 
 PKGMGR=""
 SUDO=""