chore: update golangci-lint to v2.12.2 with canonical config (#54)

Canonical v2-schema `.golangci.yml`, golangci-lint pins bumped to v2.12.2 in `Dockerfile` and `script/bootstrap`, and the tree brought to `0 issues.` under it.

Three behaviour deltas: `Cache.StoreVariant` takes a context (cancelled requests skip the accounting row, recovered by reconciliation); `MetadataStorage.Store` no longer leaks `.tmp-*.json` on Write/Close/Rename failure (dead-defer bug fix); the `signing_key` too-short error text gained a `value too short:` prefix.

Eviction-loop context cancellation deferred to #102.

internal/imgcache/stats_internal_test.go

@@ -0,0 +1,103 @@
+package imgcache
+
+import (
+	"context"
+	"database/sql"
+	"math"
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/database"
+)
+
+func setupStatsTestDB(t *testing.T) *sql.DB {
+	t.Helper()
+
+	db, err := sql.Open("sqlite", ":memory:")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = database.ApplyMigrations(context.Background(), db, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	return db
+}
+
+func TestStats_HitRateIsRatio(t *testing.T) {
+	t.Parallel()
+
+	db := setupStatsTestDB(t)
+	dir := t.TempDir()
+
+	cache, err := NewCache(db, CacheConfig{
+		StateDir:    dir,
+		CacheTTL:    time.Hour,
+		NegativeTTL: 5 * time.Minute,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ctx := context.Background()
+
+	// Set some hit/miss counts and a transform_count
+	_, err = db.ExecContext(ctx, `
+		UPDATE cache_stats
+		SET hit_count = 75, miss_count = 25, transform_count = 9999
+		WHERE id = 1
+	`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	stats, err := cache.Stats(ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if stats.HitCount != 75 {
+		t.Errorf("HitCount = %d, want 75", stats.HitCount)
+	}
+
+	if stats.MissCount != 25 {
+		t.Errorf("MissCount = %d, want 25", stats.MissCount)
+	}
+
+	// HitRate should be 0.75, NOT 9999 (transform_count)
+	expectedRate := 0.75
+	if math.Abs(stats.HitRate-expectedRate) > 0.001 {
+		t.Errorf("HitRate = %f, want %f (was it scanning transform_count?)",
+			stats.HitRate, expectedRate)
+	}
+}
+
+func TestStats_ZeroCounts(t *testing.T) {
+	t.Parallel()
+
+	db := setupStatsTestDB(t)
+	dir := t.TempDir()
+
+	cache, err := NewCache(db, CacheConfig{
+		StateDir:    dir,
+		CacheTTL:    time.Hour,
+		NegativeTTL: 5 * time.Minute,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	stats, err := cache.Stats(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// With zero hits and misses, HitRate should be 0, not some garbage value
+	if stats.HitRate != 0.0 {
+		t.Errorf("HitRate = %f, want 0.0 for zero counts", stats.HitRate)
+	}
+}