lint: adopt org-standard .golangci.yml and golangci-lint v2.12.2 (closes #14)

backend/.golangci.yml declared version: "2" on line 1 but used the
golangci-lint v1 schema below it: a top-level linters-settings key and
an issues.exclude-use-default key that does not exist in v2. Under v2
that config does not validate, so every threshold in it was inert --
lll fell back to its 120-column default rather than the intended 88,
and funlen, cyclop and dupl were not applied at all. The `0 issues.`
result the repo has been relying on was therefore meaningless.

Replace it with the org-standard file verbatim (sha256
021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb) and
move the lint stage of Dockerfile.backend from golangci-lint v2.7.2 to
the org-standard v2.12.2 by repinning the golangci/golangci-lint image
digest:

  sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240

The digest was resolved from the v2.12.2 tag and verified by running
the linter out of that exact image, which reports "golangci-lint has
version 2.12.2 built with go1.26.2 from c0d3ddc9 on
2026-05-06T11:07:58Z".

Guard against the config drifting from the standard again by asserting
its sha256 as the first step of the backend lint target. The check is a
local hash comparison against a constant in the Makefile: it needs no
network, fetches nothing, and adds no unpinned external reference to
the build path. It also catches a strictly larger class of breakage
than schema validation would, since a schema-valid but non-canonical
config is exactly how this file got into its broken state.

With the config actually loading, lll reports server.go:65 at 93
columns. Fix it, plus the two other over-long lines called out on the
issue (server.go:97 at 81 and reportbuf.go:166 at 88) which are inside
the 88-column lint limit but over the 77-column hard wrap in the Go
styleguide. All three were long //nolint justifications on the code
line; move the justification into a preceding comment block and leave
a short directive behind. No suppression is added or widened, and
.golangci.yml is not touched after the copy.

Drop the //nolint:wsl in server.go entirely rather than relocating it.
The standard config disables wsl, so the directive suppressed nothing;
removing it still yields `0 issues.`

Verified: `docker build --no-cache -f Dockerfile.backend .` is green,
with the lint stage running uncached for 14.9s and reporting
`0 issues.` -- the first proof that v2.12.2 and the canonical config
pass together through the multistage lint stage. Root `make check` and
`cd backend && make check` both pass. The drift guard's failure path
was re-proved inside the lint image: appending a byte to
.golangci.yml fails `make lint` at 0.17s with the expected and actual
hashes, before golangci-lint runs at all.

golangci-lint v2.12.2 emits a deprecation warning for the gomodguard
linter the standard config enables. It is a warning, not a failure,
and the config is not editable here; it is tracked separately.

backend/Makefile

@@ -10,6 +10,17 @@ GOLDFLAGS += -s -w
 GOLDFLAGS += -X main.Version=$(VERSION)
 GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
 
+# macOS ships shasum rather than sha256sum.
+SHA256SUM := $(shell command -v sha256sum >/dev/null 2>&1 && echo sha256sum || echo shasum -a 256)
+
+# .golangci.yml is standardized org-wide and must never be edited here
+# (REPO_POLICIES.md). Its last silent drift replaced the v2 schema with
+# v1 keys, which left every threshold in the file inert while the build
+# stayed green. The lint target therefore asserts the file still matches
+# the canonical copy byte for byte. The check is a local hash comparison:
+# no network, no remote schema, nothing unpinned in the build path.
+GOLANGCI_CONFIG_SHA256 := 021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb
+
 .PHONY: all build test lint fmt fmt-check check docker hooks run clean
 
 all: build
@@ -22,6 +33,14 @@ test:
 	timeout 30 go test ./...
 
 lint:
+	@actual=$$($(SHA256SUM) .golangci.yml | cut -d' ' -f1); \
+	if [ "$$actual" != "$(GOLANGCI_CONFIG_SHA256)" ]; then \
+		echo ".golangci.yml has drifted from the org standard."; \
+		echo "  expected $(GOLANGCI_CONFIG_SHA256)"; \
+		echo "  actual   $$actual"; \
+		echo "Restore it verbatim from sneak/prompts; do not edit it."; \
+		exit 1; \
+	fi
 	golangci-lint run ./...
 
 fmt: