chore: update golangci-lint to v2.12.2 with canonical config

Replace .golangci.yml with the canonical v2-schema config
(default: all minus six disabled linters, lll 88, tests included)
and bump every golangci-lint pin to v2.12.2:

- Dockerfile: golangci/golangci-lint:v2.12.2-alpine (hash-pinned)
- script/bootstrap: GOLANGCI_LINT_VERSION 2.12.2 with new
  linux-amd64/arm64 release-archive sha256 pins

Fix all 747 findings the stricter config surfaces, with no behavior
changes: t.Parallel() throughout the test suite, static sentinel
errors and errors.Is comparisons, checked error returns, context
propagation (contextcheck/noctx), 88-column wrapping, extracted
constants and helpers for goconst/dupl/funlen/cyclop, exhaustive
switch cases replicating existing defaults, and white-box test files
renamed to *_internal_test.go for testpackage. Three
nolint:tagliatelle directives preserve the existing snake_case JSON
wire and on-disk metadata formats.

internal/imgcache/divzero_internal_test.go

@@ -0,0 +1,47 @@
+package imgcache
+
+import (
+	"fmt"
+	"math"
+	"testing"
+)
+
+func TestSizePercentSafeWithZeroFetchBytes(t *testing.T) {
+	t.Parallel()
+
+	// Simulate the calculation from processAndStore
+	fetchBytes := int64(0)
+	outputSize := int64(100)
+
+	var sizePercent float64
+	if fetchBytes > 0 {
+		sizePercent = float64(outputSize) / float64(fetchBytes) * 100.0
+	}
+
+	// sizePercent should be 0, not Inf or NaN
+	if math.IsInf(sizePercent, 0) || math.IsNaN(sizePercent) {
+		t.Errorf("sizePercent = %f, should not be Inf/NaN", sizePercent)
+	}
+
+	// Should produce valid log output
+	result := fmt.Sprintf("%.1f%%", sizePercent)
+	if result != "0.0%" {
+		t.Errorf("formatted size ratio = %q, want %q", result, "0.0%")
+	}
+}
+
+func TestSizePercentNormalCase(t *testing.T) {
+	t.Parallel()
+
+	fetchBytes := int64(1000)
+	outputSize := int64(500)
+
+	var sizePercent float64
+	if fetchBytes > 0 {
+		sizePercent = float64(outputSize) / float64(fetchBytes) * 100.0
+	}
+
+	if math.Abs(sizePercent-50.0) > 0.001 {
+		t.Errorf("sizePercent = %f, want 50.0", sizePercent)
+	}
+}