refactor: establish pre-release schema baseline

[MartinKalema]

What changed

• Replaced the unused development migration chain with one migration that creates the current database schema.
• Removed code that translated old event messages, old dead-letter messages, and old configuration names.
• Kept one current event format and one current set of configuration names.
• Updated CI to build a fresh PostgreSQL database from the new migration and verify that the migration matches the application models.
• Updated the setup and deployment documentation.

Why

This project is not live yet, so there is no production database or deployed consumer that needs an upgrade path. Keeping compatibility code for formats that were never released makes the system harder to understand and easier to break.

The current design is now the starting standard.

Impact

Existing local development databases must be dropped and recreated. This change intentionally does not support upgrading those databases in place.

Checks completed

• 297 Python tests passed locally
• mypy passed for 161 source files
• Python bytecode compilation passed
• Alembic reports one migration head: baseline_20260712
• Docker Compose configuration validation passed
• Migration structure was compared with all current SQLAlchemy tables, indexes, and named constraints

GitHub Actions will run the full suite against a fresh migrated PostgreSQL database before merge.