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/sourceurl_internal_test.go

@@ -0,0 +1,52 @@
+package imgcache
+
+import "testing"
+
+func TestImageRequest_SourceURL_DefaultHTTPS(t *testing.T) {
+	t.Parallel()
+
+	req := &ImageRequest{
+		SourceHost:  testHostCDN,
+		SourcePath:  testPathCat,
+		SourceQuery: "v=2",
+	}
+
+	got := req.SourceURL()
+
+	want := "https://cdn.example.com/photos/cat.jpg?v=2"
+	if got != want {
+		t.Errorf("SourceURL() = %q, want %q", got, want)
+	}
+}
+
+func TestImageRequest_SourceURL_AllowHTTP(t *testing.T) {
+	t.Parallel()
+
+	req := &ImageRequest{
+		SourceHost: "localhost:8080",
+		SourcePath: testPathCat,
+		AllowHTTP:  true,
+	}
+
+	got := req.SourceURL()
+
+	want := "http://localhost:8080/photos/cat.jpg"
+	if got != want {
+		t.Errorf("SourceURL() = %q, want %q", got, want)
+	}
+}
+
+func TestImageRequest_SourceURL_AllowHTTPFalse(t *testing.T) {
+	t.Parallel()
+
+	req := &ImageRequest{
+		SourceHost: testHostCDN,
+		SourcePath: "/img.jpg",
+		AllowHTTP:  false,
+	}
+
+	got := req.SourceURL()
+	if got != "https://cdn.example.com/img.jpg" {
+		t.Errorf("SourceURL() = %q, want https scheme", got)
+	}
+}