next -> main (1.0.0 milestone) (#105)

Accumulating milestone branch. One squashed commit per closed issue; `next` is kept green and mergeable to `main` at any time without notice.

Landed so far:

- `chore: update golangci-lint to v2.12.2 with canonical config` (#54) — canonical v2-schema `.golangci.yml`, pins bumped in `Dockerfile` and `script/bootstrap`, tree at `0 issues.`. Three behaviour deltas are recorded in that PR's body: `Cache.StoreVariant` takes a context, `MetadataStorage.Store` no longer leaks temp files on failure, and the `signing_key` too-short error text gained a `value too short:` prefix.

Sequencing for the milestone is tracked in #103.

Reviewed-on: #105
Co-authored-by: clawbot <clawbot@noreply.example.org>

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)
+	}
+}