feat: Add subtitle support in OPDSEntry and enhance metadata handling in API routes

tests/test_opds_import_metadata_mapping.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from abogen.web.routes.api import _opds_metadata_overrides
+
+
+def test_opds_metadata_overrides_maps_author_and_subtitle() -> None:
+    overrides = _opds_metadata_overrides(
+        {
+            "authors": ["Alexandre Dumas"],
+            "subtitle": "Unabridged",
+            "series": "Example",
+            "series_index": 2,
+            "tags": ["Fiction", "Classic"],
+            "summary": "Summary text",
+        }
+    )
+
+    assert overrides["authors"] == "Alexandre Dumas"
+    assert overrides["author"] == "Alexandre Dumas"
+    assert overrides["subtitle"] == "Unabridged"
+
+    # Existing behavior still present
+    assert overrides["series"] == "Example"
+    assert overrides["series_index"] == "2"
+    assert overrides["tags"] == "Fiction, Classic"
+    assert overrides["description"] == "Summary text"
+
+
+def test_opds_metadata_overrides_accepts_author_string() -> None:
+    overrides = _opds_metadata_overrides({"author": "Mary Shelley"})
+    assert overrides["authors"] == "Mary Shelley"
+    assert overrides["author"] == "Mary Shelley"