feat(ocr): Add docTR OCR engine with metrics infrastructure

Add docTR as primary OCR engine with 2-tier sequential processing,
OCR metrics tracking, and simplified engine selection.

Features:
- docTR OCR engine with light+medium preprocessing tiers
- doctr_plus mode with early exit optimization (~65% fast path)
- OCR metrics dashboard with per-engine statistics
- User OCR preference persistence
- Parallel worker pool for OCR processing
- Cross-validation for extraction quality

Engine options: tesseract, doctr, doctr_plus (recommended), paddleocr

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/modules/data_entry/config.py

@@ -48,6 +48,11 @@ class Settings(BaseSettings):
     # CORS
     cors_origins: str = "http://localhost:3010,http://localhost:3000"
 
+    # OCR Engines (comma-separated list of active engines shown in UI)
+    # Available: tesseract, paddleocr, doctr, doctr_plus
+    # doctr_plus is recommended (2-tier sequential with early exit)
+    ocr_active_engines: str = "doctr,doctr_plus"
+
     class Config:
         env_file = ".env"
         env_file_encoding = "utf-8"
@@ -80,6 +85,11 @@ class Settings(BaseSettings):
         """Get CORS origins as list."""
         return [origin.strip() for origin in self.cors_origins.split(",")]
 
+    @property
+    def ocr_active_engines_list(self) -> List[str]:
+        """Get OCR active engines as list."""
+        return [engine.strip() for engine in self.ocr_active_engines.split(",")]
+
     @property
     def oracle_dsn(self) -> str:
         """Get Oracle DSN string."""