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/__init__.py

@@ -0,0 +1,27 @@
+"""OCR engine abstraction layer.
+
+Provides a pluggable engine interface for OCR processing,
+decoupling extractors from specific OCR libraries.
+"""
+
+from app.engines.base_engine import (
+    EngineError,
+    EngineProcessingError,
+    EngineUnavailableError,
+    OcrConfig,
+    OcrEngine,
+    OcrEngineResult,
+    WordBox,
+)
+from app.engines.engine_factory import create_engine
+
+__all__ = [
+    "OcrEngine",
+    "OcrConfig",
+    "OcrEngineResult",
+    "WordBox",
+    "EngineError",
+    "EngineUnavailableError",
+    "EngineProcessingError",
+    "create_engine",
+]