Update golangci-lint to v2.12.2 with canonical config (#187)

Bumps golangci-lint from v2.10.1 to v2.12.2 everywhere it is pinned and installs the canonical `.golangci.yml`, then fixes every finding the new linter surfaces so `make check` is green.

## Version pins

- `Dockerfile` lint stage: `golangci/golangci-lint:v2.12.2` (Debian-based), tag plus digest pin
- `script/bootstrap`: `GOLANGCI_LINT_VERSION=2.12.2` with updated `linux-amd64`/`linux-arm64` release-archive sha256 pins

## Config

`.golangci.yml` replaced with the canonical config. Material change: the old file declared `version: "2"` but kept settings under the legacy top-level `linters-settings` key, which golangci-lint v2 ignores — so the intended thresholds (`lll` 88, `funlen` 80/50, `cyclop` 15, `dupl` 100) were not being applied. The canonical file moves them under `linters.settings` and drops `issues.exclude-use-default`.

## Lint fixes (216 findings)

- `lll` (96): wrapped lines to the 88-column limit
- `noctx` (46): `httptest.NewRequestWithContext` with `t.Context()` throughout the tests
- `goconst` (24): shared constants for template/JSON keys in `internal/handlers` and repeated test literals
- `gosec` (23): app-page redirects now go through a `redirectToApp` helper that path-escapes the app ID (G710 open redirect); `http.ServeFile` of the internally derived deployment log path annotated like the adjacent `os.Stat` (G703)
- `dupl` (22): extracted a generic `findAllByAppID` in `internal/models`, a `deleteAppResource` helper in `internal/handlers`, a shared `parsePush` in `internal/service/webhook`, and table-driven/helper-based dedup in tests
- `nolintlint` (5): removed `//nolint:funlen` directives made obsolete by the new limits (plus one more that became obsolete after refactoring)
- `nilerr` (3, surfaced during fixing): resource-delete lookups now propagate the find error to the caller

No behavior changes intended; all tests pass and `make check` is green.

Note: golangci-lint v2.12 warns that `gomodguard` is deprecated in favor of `gomodguard_v2` — a future canonical-config update should address this centrally.
Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #187
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

TODO.md

@@ -10,18 +10,20 @@
 
 # Status
 
-1.0+. Tagged 1.0.0 on 2026-02-26; 8 commits on main since. Policy
-violation: main currently fails make check (91 lint issues), so the tree
-is out of compliance until fixed.
+1.0+. Tagged 1.0.0 on 2026-02-26; 8 commits on main since. `make check`
+is green as of the golangci-lint v2.12.2 update.
 
 # Next Step
 
-Fix the 47 noctx lint findings (HTTP requests without context) in one
-commit and confirm the count drops under make check. This is the largest
-of the three lint classes blocking a green main.
+Confirm `.gitea/workflows/check.yml` gates merges on `make check` so
+main cannot regress.
 
 # Completed Steps
 
+- 2026-08-07: Updated golangci-lint to v2.12.2 (canonical
+  `.golangci.yml`, `Dockerfile` lint stage pin, `script/bootstrap`
+  release-archive pins) and fixed all resulting lint findings (noctx,
+  gosec, goconst, lll, dupl, nolintlint); `make check` green.
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
   Makefile shims, README Entrypoints section
 - 2026-03-11: Monolithic env var editing with bulk save (#158).
@@ -44,12 +46,4 @@ of the three lint classes blocking a green main.
 
 # Future Steps
 
-- Get main green (compliance, ordered):
-    - Fix 47 noctx findings (Next Step).
-    - Fix 23 gosec findings.
-    - Fix 21 goconst findings.
-    - Run make check clean on main and keep it green; main must always
-      pass.
-- Confirm .gitea/workflows/check.yml gates merges on make check so main
-  cannot regress.
 - Resume feature work only after main is green.