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>

entrypoint.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+set -e  # Exit on any error
+
+echo "Running database migrations..."
+if uv run alembic upgrade head; then
+    echo "Database migrations completed successfully"
+else
+    echo "ERROR: Database migration failed!"
+    echo "Please check the logs above for details."
+    exit 1
+fi
+
+echo "Starting application server..."
+exec gunicorn --bind 0.0.0.0:8000 --workers 2 --threads 4 main:app