Store SHA256 checksums with artifacts and add multiple hash support

- Add sha256 field to API responses as explicit alias of artifact id
- Add checksum_sha1 and s3_etag fields to artifacts table
- Compute MD5, SHA1, and capture S3 ETag during upload
- Update StorageResult to return all checksums from storage layer
- Add migration 003_checksum_fields.sql for existing databases
- Add Dockerfile.local and docker-compose.local.yml for local development
- Update schemas to include all checksum fields in responses

Dockerfile.local

@@ -0,0 +1,50 @@
+# Frontend build stage
+FROM node:20-alpine AS frontend-builder
+
+WORKDIR /app/frontend
+
+# Copy package files
+COPY frontend/package*.json ./
+RUN npm install
+
+# Copy frontend source
+COPY frontend/ ./
+
+# Build frontend
+RUN npm run build
+
+# Runtime stage
+FROM python:3.12-slim
+
+# Install system dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
+
+# Create non-root user
+RUN groupadd -g 1000 orchard && \
+    useradd -u 1000 -g orchard -s /bin/bash -m orchard
+
+WORKDIR /app
+
+# Copy requirements and install Python dependencies
+COPY backend/requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy backend source
+COPY backend/ ./backend/
+
+# Copy frontend build
+COPY --from=frontend-builder /app/frontend/dist ./frontend/dist
+
+# Set ownership
+RUN chown -R orchard:orchard /app
+
+USER orchard
+
+EXPOSE 8080
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+    CMD curl -f http://localhost:8080/health || exit 1
+
+CMD ["uvicorn", "backend.app.main:app", "--host", "0.0.0.0", "--port", "8080"]