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

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="2.11.3"
-# sha256 of golangci-lint-2.11.3-linux-<arch>.tar.gz release archives
-GOLANGCI_LINT_SHA256_AMD64="87bb8cddbcc825d5778b64e8a91b46c0526b247f4e2f2904dea74ec7450475d1"
-GOLANGCI_LINT_SHA256_ARM64="ee3d95f301359e7d578e6d99c8ad5aeadbabc5a13009a30b2b0df11c8058afe9"
+# 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=""