feat: add Participant and MagicToken models with automatic migrations

Implements Phase 2 infrastructure for participant registration and authentication:

Database Models:
- Add Participant model with exchange scoping and soft deletes
- Add MagicToken model for passwordless authentication
- Add participants relationship to Exchange model
- Include proper indexes and foreign key constraints

Migration Infrastructure:
- Generate Alembic migration for new models
- Create entrypoint.sh script for automatic migrations on container startup
- Update Containerfile to use entrypoint script and include uv binary
- Remove db.create_all() in favor of migration-based schema management

This establishes the foundation for implementing stories 4.1-4.3, 5.1-5.3, and 10.1.

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

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

Containerfile

@@ -23,11 +23,18 @@ RUN useradd --create-home --shell /bin/bash appuser
 # Copy virtual environment from builder
 COPY --from=builder /app/.venv /app/.venv
 
+# Copy uv for running alembic in entrypoint
+COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv
+
 # Copy application code
 COPY src/ ./src/
 COPY migrations/ ./migrations/
 COPY alembic.ini main.py ./
 
+# Copy entrypoint script
+COPY entrypoint.sh ./
+RUN chmod +x entrypoint.sh
+
 # Set environment variables
 ENV PATH="/app/.venv/bin:$PATH"
 ENV PYTHONUNBUFFERED=1
@@ -47,5 +54,5 @@ EXPOSE 8000
 HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 \
     CMD python -c "import urllib.request; urllib.request.urlopen('http://localhost:8000/health')" || exit 1
 
-# Run with gunicorn
-CMD ["gunicorn", "--bind", "0.0.0.0:8000", "--workers", "2", "--threads", "4", "main:app"]
+# Run with entrypoint script
+CMD ["./entrypoint.sh"]