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

Dockerfile

@@ -1,8 +1,8 @@
 # Lint stage
-# golangci/golangci-lint:v2.11.3 (Debian-based), 2026-03-17
+# golangci/golangci-lint:v2.12.2 (Debian-based), 2026-08-07
 # Using Debian-based image because mattn/go-sqlite3 (CGO) does not
 # compile on Alpine musl (off64_t is a glibc type).
-FROM golangci/golangci-lint:v2.11.3@sha256:e838e8ab68aaefe83e2408691510867ade9329c0e0b895a3fb35eb93d1c2a4ba AS lint
+FROM golangci/golangci-lint:v2.12.2@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
 
 RUN apt-get update && apt-get install -y --no-install-recommends make && rm -rf /var/lib/apt/lists/*