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.

TODO.md

@@ -10,18 +10,27 @@
 
 # Status
 
-pre-1.0. No git tags. Backend work in flight on feat/reportbuf-storage (dirty:
-src/main.js). Frontend is functional; backend is new and unmerged.
+pre-1.0. No git tags. `feat/reportbuf-storage` is merged; the backend, the CI
+workflow, and the backend repo standard files are all on `main`. Frontend and
+backend are both functional. Working toward the 1.0.0 milestone by closing the
+remaining repo-compliance issues on the tracker.
 
 # Next Step
 
-Land feat/reportbuf-storage: finish the in-progress src/main.js change, get make
-check green, and merge the branch to main. The branch adds the backend (buffered
-zstd-compressed report storage), the CI workflow, and backend repo standard
-files, so merging it also closes most compliance gaps.
+Confirm the `.gitea/workflows/check.yml` run is green (main always green
+policy). The workflow file is already on `main`; what is unverified is that its
+latest run passes.
 
 # Completed Steps
 
+- 2026-08-10: adopted the org-standard `backend/.golangci.yml` verbatim and
+  moved the pinned golangci-lint from v2.7.2 to v2.12.2 (the `lint` stage of
+  `Dockerfile.backend` now pins the `golangci/golangci-lint:v2.12.2` image by
+  digest); the previous config declared `version: "2"` but used v1 schema keys,
+  so every threshold in it was inert and its green result was meaningless.
+  `backend/Makefile`'s `lint` target now asserts the config's sha256 against the
+  canonical file first, so drift from the org standard fails the build instead
+  of silently degrading to defaults
 - 2026-08-10: every interactive control now meets the 44x44 CSS px minimum tap
   target (`.pin-btn`, `#interval-select`, the debug-log label and, on narrow
   viewports, `#pause-btn`). The pin button's hit area grows via matching
@@ -43,7 +52,7 @@ files, so merging it also closes most compliance gaps.
   shims, README Entrypoints section
 - 2026-02-27: backend with buffered zstd-compressed report storage; CI workflow
   and backend repo standard files; backend Dockerfile fixed (Go 1.25,
-  golangci-lint) and moved to repo root (feat/reportbuf-storage, unmerged)
+  golangci-lint) and moved to repo root (feat/reportbuf-storage)
 - 2026-02-26: host row layout redesigned with CSS grid; overflow and spacing
   fixes; nginx config extracted; port hardcoded to 8080
 - 2026-02-26: debug log panel, median stats, recovery probe, Docker build fix,
@@ -56,9 +65,11 @@ files, so merging it also closes most compliance gaps.
 
 # Future Steps
 
-- Compliance top-up as one small commit: add .editorconfig and add the hooks
-  target to the Makefile
-- After merge, confirm .gitea/workflows/check.yml is on main and CI is green
-  (main always green policy)
 - Decide what to do with untracked resume.sh: commit it, gitignore it, or delete
   it
+- Upstream fix needed in `sneak/prompts`: the org-standard `.golangci.yml`
+  enables `gomodguard`, which golangci-lint v2.12.2 reports as deprecated since
+  v2.12.0 and replaced by `gomodguard_v2`, so every backend lint run prints a
+  deprecation warning. The file is standardized and must never be edited in this
+  repo, so nothing can be done here beyond tracking it — tracked at
+  <https://git.eeqj.de/sneak/netwatch/issues/41>