Update golangci-lint to v2.12.2 with canonical config

Bump the golangci-lint Docker image pin in Dockerfile and the
release-archive sha256 pins in script/bootstrap from 2.11.3 to
2.12.2, and replace .golangci.yml with the canonical config. The
canonical config moves lll/funlen/cyclop/dupl settings from the
top-level linters-settings key (ignored by the v2 schema) to
linters.settings, so those thresholds now actually apply.

Fix all findings the newly applied thresholds surfaced:

- lll: wrap or shorten seven over-length lines (struct tag
  comments, test logger construction, a func signature, and a
  nosec comment)
- goconst: use http.MethodPost/http.MethodPut and new shared
  constants for repeated test strings; add tmplKeyError and
  tmplKeyWebhook constants for template data keys in handlers
- dupl: merge buildHTTPTargetConfig and buildSlackTargetConfig
  into a parameterized buildURLTargetConfig; drop the duplicate
  iWebhookDB test helper in favor of testWebhookDB; extract
  shared helpers in middleware and session tests

TODO.md

@@ -28,6 +28,11 @@ databases currently grow without bound.
 
 # Completed Steps
 
+- 2026-08-07 Update golangci-lint to v2.12.2 (Docker image digest in
+  `Dockerfile`, release-archive sha256 pins in `script/bootstrap`),
+  adopt the canonical `.golangci.yml` (v2 `linters.settings` layout so
+  `lll`/`funlen`/`cyclop`/`dupl` thresholds actually apply), and fix
+  all newly surfaced lint findings
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
   Makefile shims, README Entrypoints section
 - 2026-03-25 pin golangci-lint Docker image for linting (#55)