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/negative_cache_internal_test.go

@@ -0,0 +1,109 @@
+package imgcache
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+)
+
+func TestNegativeCache_StoreAndCheck(t *testing.T) {
+	t.Parallel()
+
+	db := setupTestDB(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()
+	req := &ImageRequest{
+		SourceHost: testHostExample,
+		SourcePath: "/missing.jpg",
+	}
+
+	// Initially should not be in negative cache
+	hit, err := cache.checkNegativeCache(ctx, req)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if hit {
+		t.Error("expected no negative cache hit initially")
+	}
+
+	// Store a negative entry
+	err = cache.StoreNegative(ctx, req, 404, "not found")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Now should be in negative cache
+	hit, err = cache.checkNegativeCache(ctx, req)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !hit {
+		t.Error("expected negative cache hit after storing")
+	}
+}
+
+func TestNegativeCache_Expired(t *testing.T) {
+	t.Parallel()
+
+	db := setupTestDB(t)
+	dir := t.TempDir()
+
+	cache, err := NewCache(db, CacheConfig{
+		StateDir:    dir,
+		CacheTTL:    time.Hour,
+		NegativeTTL: 1 * time.Millisecond, // very short TTL
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	ctx := context.Background()
+	req := &ImageRequest{
+		SourceHost: testHostExample,
+		SourcePath: "/expired.jpg",
+	}
+
+	// Store a negative entry with very short TTL
+	err = cache.StoreNegative(ctx, req, 500, "server error")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Wait for expiry
+	time.Sleep(10 * time.Millisecond)
+
+	// Should no longer be in negative cache
+	hit, err := cache.checkNegativeCache(ctx, req)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if hit {
+		t.Error("expected expired negative cache entry to be a miss")
+	}
+}
+
+func TestService_Get_ReturnsErrorForNegativeCachedURL(t *testing.T) {
+	t.Parallel()
+
+	// This test verifies that Service.Get() checks the negative cache
+	// We can't easily test the full pipeline without vips, but we can
+	// verify the error type
+	err := ErrNegativeCached
+	if !errors.Is(err, ErrNegativeCached) {
+		t.Error("ErrNegativeCached should be identifiable with errors.Is")
+	}
+}