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/Dockerfile

@@ -0,0 +1,23 @@
+FROM python:3.11-slim
+
+# System dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    tesseract-ocr \
+    tesseract-ocr-eng \
+    libtesseract-dev \
+    libheif1 \
+    libheif-dev \
+    libglib2.0-0 \
+    libmagic1 \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# Python dependencies
+WORKDIR /app
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY . .
+
+EXPOSE 8000
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]