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/app/config.py

@@ -0,0 +1,15 @@
+"""OCR Service Configuration."""
+import os
+
+
+class Settings:
+    """Application settings loaded from environment variables."""
+
+    def __init__(self) -> None:
+        self.log_level: str = os.getenv("LOG_LEVEL", "info")
+        self.host: str = os.getenv("HOST", "0.0.0.0")
+        self.port: int = int(os.getenv("PORT", "8000"))
+        self.tesseract_cmd: str = os.getenv("TESSERACT_CMD", "/usr/bin/tesseract")
+
+
+settings = Settings()