Remove runtime DB initialization; require explicit Alembic migrations

[AlexKitipov]

Motivation

• Prevent the application from creating or modifying the database schema at runtime so production schema changes are applied only via migrations.
• Make schema migrations an explicit step in deployment and container workflows to avoid accidental in-place schema drift.

Description

• Removed runtime database initialization from the FastAPI startup path by deleting the init_db import and the lifespan hook in app/main.py, so the app no longer calls create_all on startup.
• Added scripts/migrate.sh which runs alembic upgrade head and copied the scripts/ directory into the Docker image via Dockerfile so migrations can be run from the image.
• Added a migrate service to docker-compose.yml (profile migrations) that runs the migration helper as a separate step, and removed the implicit alembic upgrade head from the backend startup command.
• Updated deploy.sh and README.md to ensure .env is created before migrations and to run migrations explicitly (./scripts/migrate.sh / alembic upgrade head) before enabling/starting the service or bringing up the API.

Testing

• Ran python -m pytest and all tests passed (8 passed).
• Ran git diff --check which reported no issues.
• Performed static shell checks with sh -n scripts/migrate.sh deploy.sh which succeeded without syntax errors.
• Attempted docker compose config validation but it could not be run in this environment because Docker is not installed.

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