Update golangci-lint to v2.12.2 with canonical config (#187)

Bumps golangci-lint from v2.10.1 to v2.12.2 everywhere it is pinned and installs the canonical `.golangci.yml`, then fixes every finding the new linter surfaces so `make check` is green.

## Version pins

- `Dockerfile` lint stage: `golangci/golangci-lint:v2.12.2` (Debian-based), tag plus digest pin
- `script/bootstrap`: `GOLANGCI_LINT_VERSION=2.12.2` with updated `linux-amd64`/`linux-arm64` release-archive sha256 pins

## Config

`.golangci.yml` replaced with the canonical config. Material change: the old file declared `version: "2"` but kept settings under the legacy top-level `linters-settings` key, which golangci-lint v2 ignores — so the intended thresholds (`lll` 88, `funlen` 80/50, `cyclop` 15, `dupl` 100) were not being applied. The canonical file moves them under `linters.settings` and drops `issues.exclude-use-default`.

## Lint fixes (216 findings)

- `lll` (96): wrapped lines to the 88-column limit
- `noctx` (46): `httptest.NewRequestWithContext` with `t.Context()` throughout the tests
- `goconst` (24): shared constants for template/JSON keys in `internal/handlers` and repeated test literals
- `gosec` (23): app-page redirects now go through a `redirectToApp` helper that path-escapes the app ID (G710 open redirect); `http.ServeFile` of the internally derived deployment log path annotated like the adjacent `os.Stat` (G703)
- `dupl` (22): extracted a generic `findAllByAppID` in `internal/models`, a `deleteAppResource` helper in `internal/handlers`, a shared `parsePush` in `internal/service/webhook`, and table-driven/helper-based dedup in tests
- `nolintlint` (5): removed `//nolint:funlen` directives made obsolete by the new limits (plus one more that became obsolete after refactoring)
- `nilerr` (3, surfaced during fixing): resource-delete lookups now propagate the find error to the caller

No behavior changes intended; all tests pass and `make check` is green.

Note: golangci-lint v2.12 warns that `gomodguard` is deprecated in favor of `gomodguard_v2` — a future canonical-config update should address this centrally.
Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #187
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/service/deploy/export_test.go

@@ -26,7 +26,11 @@ func (svc *Service) CancelActiveDeploy(appID string) {
 }
 
 // RegisterActiveDeploy registers an active deploy for testing.
-func (svc *Service) RegisterActiveDeploy(appID string, cancel context.CancelFunc, done chan struct{}) {
+func (svc *Service) RegisterActiveDeploy(
+	appID string,
+	cancel context.CancelFunc,
+	done chan struct{},
+) {
 	svc.activeDeploys.Store(appID, &activeDeploy{cancel: cancel, done: done})
 }
 
@@ -41,7 +45,11 @@ func (svc *Service) UnlockApp(appID string) {
 }
 
 // NewTestServiceWithConfig creates a Service with config and docker client for testing.
-func NewTestServiceWithConfig(log *slog.Logger, cfg *config.Config, dockerClient *docker.Client) *Service {
+func NewTestServiceWithConfig(
+	log *slog.Logger,
+	cfg *config.Config,
+	dockerClient *docker.Client,
+) *Service {
 	return &Service{
 		log:    log,
 		config: cfg,