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

The old backend/.golangci.yml declared version "2" but used v1 schema
keys, so under v2 it never validated and its thresholds were inert: the
linter ran at defaults. Replace it verbatim with the org-standard file,
repin the Dockerfile.backend lint stage to golangci-lint v2.12.2, and
assert the config's sha256 as the first step of the backend lint target
so it cannot silently drift again -- a local hash check, no network.

The standard config surfaces findings only in the tests: the repeated
IP literals in middleware_test.go become named constants (goconst) and
its request switches to NewRequestWithContext (noctx). reportbuf.go's
gosec suppression gains a plain justification comment. The rest of the
backend, including the fx-based server lifecycle, is already clean.
TODO.md updated.

Model: opus-4-8

TODO.md

@@ -10,15 +10,16 @@
 
 # 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
 
@@ -36,6 +37,14 @@ files, so merging it also closes most compliance gaps.
   frame/sniff/referrer/permissions headers) registered before CORS, and
   trusted-proxy client IP resolution honouring `X-Forwarded-For` / `X-Real-IP`
   only from a `TRUSTED_PROXIES` allowlist (loopback plus RFC1918 by default)
+- 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
@@ -65,7 +74,7 @@ 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,
@@ -87,3 +96,9 @@ files, so merging it also closes most compliance gaps.
   (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>