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

backend/internal/reportbuf/reportbuf.go

@@ -169,7 +169,9 @@ func (b *Buffer) writeFile(data []byte) {
 	name := fmt.Sprintf("reports-%s.jsonl.zst", ts)
 	path := filepath.Join(b.dataDir, name)
 
-	f, err := os.OpenFile( //nolint:gosec // path built from controlled dataDir + timestamp
+	// path is built from the operator-supplied dataDir plus a
+	// generated timestamp, so it carries no external input.
+	f, err := os.OpenFile( //nolint:gosec // see comment above
 		path,
 		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
 		filePerms,