feat(ocr): Implement persistent worker pool with SQLite job queue

Major OCR infrastructure improvements:
- Add persistent SQLite-based job queue for OCR tasks
- Implement worker pool with process isolation and auto-restart
- Add OCR engine selector dropdown (Tesseract/PaddleOCR) in upload zone
- Optimize Tesseract preprocessing based on benchmark results (8x faster)
- Add recognize_cif_optimized() with multi-strategy CIF extraction
- Add Romanian CIF checksum validation
- Increase Telegram long polling timeout from 10s to 30s

Squashed commits:
- feat(ocr): Implement persistent worker pool with SQLite job queue
- feat(ocr): Add OCR engine selector dropdown to upload zone
- perf(telegram): Increase long polling timeout from 10s to 30s
- perf(ocr): Optimize Tesseract preprocessing based on benchmark results

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

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

backend/modules/data_entry/schemas/ocr.py

@@ -136,3 +136,101 @@ class OCRStatusResponse(BaseModel):
     available: bool = Field(description="Whether OCR service is available")
     engines: list[str] = Field(description="Available OCR engines")
     message: str = Field(description="Status message")
+
+
+# ============================================================================
+# Job Queue Schemas (for async OCR processing)
+# ============================================================================
+
+from datetime import datetime
+from enum import Enum
+
+
+class OCREngineChoice(str, Enum):
+    """OCR engine selection options."""
+    auto = "auto"
+    paddleocr = "paddleocr"
+    tesseract = "tesseract"
+
+
+class OCRJobStatus(str, Enum):
+    """OCR job status."""
+    pending = "pending"
+    processing = "processing"
+    completed = "completed"
+    failed = "failed"
+
+
+class OCRJobSubmitResponse(BaseModel):
+    """Response when submitting an OCR job."""
+
+    job_id: str = Field(description="Unique job identifier (UUID)")
+    status: OCRJobStatus = Field(description="Initial job status (pending)")
+    queue_position: int = Field(description="Position in queue (1 = next to process)")
+    estimated_wait_seconds: int = Field(description="Estimated wait time in seconds")
+    created_at: datetime = Field(description="Job creation timestamp")
+
+    class Config:
+        """Pydantic config."""
+        json_schema_extra = {
+            "example": {
+                "job_id": "abc123-def456-ghi789",
+                "status": "pending",
+                "queue_position": 3,
+                "estimated_wait_seconds": 21,
+                "created_at": "2024-01-15T12:00:00"
+            }
+        }
+
+
+class OCRJobResponse(BaseModel):
+    """Full OCR job status response."""
+
+    job_id: str = Field(description="Unique job identifier")
+    status: OCRJobStatus = Field(description="Current job status")
+    queue_position: Optional[int] = Field(default=None, description="Queue position (None if processing/completed)")
+    estimated_wait_seconds: Optional[int] = Field(default=None, description="Estimated wait time")
+    created_at: datetime = Field(description="Job creation timestamp")
+    started_at: Optional[datetime] = Field(default=None, description="Processing start timestamp")
+    completed_at: Optional[datetime] = Field(default=None, description="Completion timestamp")
+    processing_time_ms: Optional[int] = Field(default=None, description="Actual processing time in ms")
+    result: Optional[ExtractionData] = Field(default=None, description="Extraction result (only if completed)")
+    error: Optional[str] = Field(default=None, description="Error message (only if failed)")
+
+    class Config:
+        """Pydantic config."""
+        json_schema_extra = {
+            "example": {
+                "job_id": "abc123-def456-ghi789",
+                "status": "completed",
+                "queue_position": None,
+                "estimated_wait_seconds": 0,
+                "created_at": "2024-01-15T12:00:00",
+                "started_at": "2024-01-15T12:00:21",
+                "completed_at": "2024-01-15T12:00:28",
+                "processing_time_ms": 6543,
+                "result": {
+                    "receipt_number": "123",
+                    "amount": 85.99,
+                    "ocr_engine": "paddleocr-light"
+                }
+            }
+        }
+
+
+class OCRQueueStatusResponse(BaseModel):
+    """Queue statistics response."""
+
+    pending_jobs: int = Field(description="Number of jobs waiting in queue")
+    processing_jobs: int = Field(description="Number of jobs currently processing")
+    average_time_seconds: float = Field(description="Average processing time in seconds")
+
+    class Config:
+        """Pydantic config."""
+        json_schema_extra = {
+            "example": {
+                "pending_jobs": 5,
+                "processing_jobs": 1,
+                "average_time_seconds": 7.2
+            }
+        }