chore(backend): re-vendor .golangci.yml with gomodguard_v2 (closes #41)

golangci-lint v2.12 deprecates gomodguard, which the org .golangci.yml
reached through "default: all", so every lint run printed a
deprecation warning. backend/.golangci.yml is now the current copy
from sneak/prompts, fetched unedited: gomodguard is disabled and
gomodguard_v2 enabled with the org block list. The new file also
turns depguard on with its test-support rule, which forbids
net/http/httptest outside test code. netwatch has no test-support
packages of its own to add to that rule, so the file is identical to
the canonical one. backend/script/lint checks the new sha256. The
backend raises no findings under the new rules.

Model: opus-5-5

backend/script/lint

@@ -14,7 +14,7 @@ set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-GOLANGCI_CONFIG_SHA256="021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb"
+GOLANGCI_CONFIG_SHA256="a79b63a254602a5318db5d0e9a06bc71b84bf0c1d896305229d8bfed1d1b1776"
 
 main() {
     cd "$ROOT"