feat: add OCR engine abstraction layer (refs #116)

Introduce pluggable OcrEngine ABC with PaddleOCR PP-OCRv4 as primary
engine and Tesseract wrapper for backward compatibility. Engine factory
reads OCR_PRIMARY_ENGINE config to instantiate the correct engine.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ocr/app/engines/engine_factory.py

@@ -0,0 +1,52 @@
+"""Factory function for creating OCR engine instances from configuration."""
+
+import logging
+
+from app.config import settings
+from app.engines.base_engine import EngineUnavailableError, OcrEngine
+
+logger = logging.getLogger(__name__)
+
+# Valid engine identifiers
+_ENGINE_REGISTRY: dict[str, str] = {
+    "paddleocr": "app.engines.paddle_engine.PaddleOcrEngine",
+    "tesseract": "app.engines.tesseract_engine.TesseractEngine",
+}
+
+
+def create_engine(engine_name: str | None = None) -> OcrEngine:
+    """Instantiate an OCR engine by name (defaults to config value).
+
+    Args:
+        engine_name: Engine identifier ("paddleocr", "tesseract").
+                     Falls back to ``settings.ocr_primary_engine``.
+
+    Returns:
+        Initialized OcrEngine instance.
+
+    Raises:
+        EngineUnavailableError: If the engine cannot be loaded or initialized.
+    """
+    name = (engine_name or settings.ocr_primary_engine).lower().strip()
+
+    if name not in _ENGINE_REGISTRY:
+        raise EngineUnavailableError(
+            f"Unknown engine '{name}'. Available: {list(_ENGINE_REGISTRY.keys())}"
+        )
+
+    module_path, class_name = _ENGINE_REGISTRY[name].rsplit(".", 1)
+
+    try:
+        import importlib
+
+        module = importlib.import_module(module_path)
+        engine_cls = getattr(module, class_name)
+        engine: OcrEngine = engine_cls()
+        logger.info("Created OCR engine: %s", name)
+        return engine
+    except EngineUnavailableError:
+        raise
+    except Exception as exc:
+        raise EngineUnavailableError(
+            f"Failed to create engine '{name}': {exc}"
+        ) from exc