build: update golangci-lint to v2.12.2 with canonical config

Replace the commit-pinned golangci-lint install refs in Dockerfile and
script/bootstrap with the v2.12.2 version pin, and replace .golangci.yml
with the canonical v2-schema config (lint settings now live under
linters.settings, so the lll/funlen/cyclop/dupl thresholds are actually
applied).

Fix all findings surfaced by the newly-applied config:

- goconst: introduce shared constants for repeated status, priority,
  and DNS fixture strings in watcher.go and the notify, state, and
  watcher tests
- dupl: consolidate duplicated ntfy/slack HTTP-error tests and
  SendNotification endpoint-error tests behind shared helpers
- lll: wrap long test table entries and comments; shorten one inline
  nolint justification

script/bootstrap

@@ -4,14 +4,14 @@
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes nothing is present.
 # golangci-lint and goimports are installed via `go install` at the same
-# pinned commits the Dockerfile uses (never "latest").
+# pinned refs the Dockerfile uses (never "latest").
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-07-07 (same pins as the Dockerfile)
-# golangci-lint v2.10.1
-GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@5d1e709b7be35cb2025444e19de266b056b7b7ee"
+# Pinned versions, 2026-08-07 (same pins as the Dockerfile)
+# golangci-lint v2.12.2
+GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2"
 # goimports v0.42.0
 GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"