feat: add OCR service container (refs #64)

Add Python-based OCR service container (mvp-ocr) as the 6th service:
- Python 3.11-slim with FastAPI/uvicorn
- Tesseract OCR with English language pack
- pillow-heif for HEIC image support
- opencv-python-headless for image preprocessing
- Health endpoint at /health
- Unit tests for health, HEIC support, and Tesseract availability

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

ocr/tests/__init__.py

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+# OCR Service Tests

ocr/tests/test_health.py

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+"""Tests for OCR service health and core functionality."""
+import io
+
+import pytest
+from fastapi.testclient import TestClient
+from PIL import Image
+
+from app.main import app
+
+
+@pytest.fixture
+def client():
+    """Create test client for FastAPI app."""
+    return TestClient(app)
+
+
+def test_health_endpoint(client):
+    """Health endpoint returns healthy status."""
+    response = client.get("/health")
+    assert response.status_code == 200
+    assert response.json() == {"status": "healthy"}
+
+
+def test_root_endpoint(client):
+    """Root endpoint returns service information."""
+    response = client.get("/")
+    assert response.status_code == 200
+    data = response.json()
+    assert data["service"] == "mvp-ocr"
+    assert "version" in data
+
+
+def test_pillow_heif_can_register():
+    """pillow-heif can register with Pillow for HEIC support."""
+    import pillow_heif
+
+    pillow_heif.register_heif_opener()
+    # Verify HEIC format is registered
+    assert "HEIF" in Image.registered_extensions().values()
+
+
+def test_tesseract_available():
+    """Tesseract OCR is available and can process images."""
+    import pytesseract
+
+    # Create a simple test image with text
+    img = Image.new("RGB", (200, 50), color="white")
+
+    # Verify pytesseract can call tesseract (will return empty string for blank image)
+    result = pytesseract.image_to_string(img)
+    # Just verify it doesn't raise an exception - blank image returns empty/whitespace
+    assert isinstance(result, str)