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>

src/app.py

@@ -72,9 +72,14 @@ def create_app(config_name: str | None = None) -> Flask:
 
     # Import models to ensure they're registered with SQLAlchemy
     with app.app_context():
-        from src.models import Admin, Exchange, RateLimit  # noqa: F401
-
-        db.create_all()
+        from src.models import (  # noqa: F401
+            Admin,
+            Exchange,
+            MagicToken,
+            Participant,
+            RateLimit,
+        )
+        # Schema managed by Alembic migrations (applied via entrypoint script)
 
     return app