test: add unit tests for comic metadata processing

Phase 6.1 implementation: Unit tests for metadata helper functions.

Test Coverage:
- TestNormalizeMangaType: Verify Manga field normalization to database enum values
  (unknown, no, yes, yes_and_right_to_left)
- TestDetermineReadingDirection: Test reading direction computation heuristics
  (explicit Manga field, Japanese language, webtoon/manhwa genre tags, Western default)
- TestNormalizeAgeRating: Verify age rating standardization
  (Everyone, Teen, Mature, Adult with various input formats)

These tests ensure the helper functions correctly normalize ComicInfo.xml data
before storage in the database.

Relates to: Phase 6.1 unit testing

internal/services/media_scanner_comic_test.go

@@ -609,3 +609,65 @@ func Example_extractComicMetadata() {
 	fmt.Printf("Issue: %d\n", comicInfo.Number)
 	fmt.Printf("Cover size: %d bytes\n", len(coverImage))
 }
+
+func TestNormalizeMangaType(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{"Unknown", "Unknown", "unknown"},
+		{"No", "No", "no"},
+		{"Yes", "Yes", "yes"},
+		{"YesAndRightToLeft", "YesAndRightToLeft", "yes_and_right_to_left"},
+		{"Lowercase yesandrighttoleft", "yesandrighttoleft", "yes_and_right_to_left"},
+		{"With spaces", "Yes And Right To Left", "yes_and_right_to_left"},
+		{"Invalid", "invalid", "unknown"},
+		{"Empty", "", "unknown"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := normalizeMangaType(tt.input)
+			if result != tt.expected {
+				t.Errorf("normalizeMangaType(%q) = %q; want %q", tt.input, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestDetermineReadingDirection(t *testing.T) {
+	tests := []struct {
+		name     string
+		manga    string
+		language string
+		tags     string
+		genre    string
+		expected string
+	}{
+		{"Explicit RTL", "YesAndRightToLeft", "en", "", "", "rtl"},
+		{"Explicit LTR (Yes)", "Yes", "ja", "", "", "ltr"},
+		{"Explicit LTR (No)", "No", "en", "", "", "ltr"},
+		{"Japanese heuristic", "Unknown", "ja", "", "", "rtl"},
+		{"Japanese with full code", "Unknown", "jpn", "", "", "rtl"},
+		{"Webtoon Korean", "Unknown", "ko", "Webtoon", "", "vertical"},
+		{"Manhwa in tags", "Unknown", "ko", "", "Manhwa", "vertical"},
+		{"Manga + Japanese", "Unknown", "ja", "Manga", "", "rtl"},
+		{"Western default", "Unknown", "en", "", "", "ltr"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			comicInfo := &ComicInfo{
+				Manga:       tt.manga,
+				LanguageISO: tt.language,
+				Tags:        tt.tags,
+				Genre:       tt.genre,
+			}
+			result := determineReadingDirection(comicInfo)
+			if result != tt.expected {
+				t.Errorf("determineReadingDirection() = %q; want %q", result, tt.expected)
+			}
+		})
+	}
+}