feat: DB optimization, improved container security and improved pipeline checks

[codewithme-py]

What has been done:

1. Database optimization (SQL Tuning)

• Added indexes (B-Tree) to the columns status and created_at in the table reservations.

Why: The ARQ background worker (who searches for overdue reservations) began to work out requests for ~0.08ms (Index Scan instead of Full Table Scan), which is critical for High-Load scenarios during product drops.

2. Docker Hardening (Container Safety)

• A system non-root user appuser has been created for the FastAPI process.

• The root file system of the container is switched to read_only: true mode via docker-compose.yaml Writing is allowed only in tmpfs (/tmp) for temporary files.

Why: Minimizing the consequences in the case of a potential RCE - the attacker will not get root rights and will not be able to overwrite the source code of the application.

3. CI / Dealing with addictions

• Environment consistency validation has been implemented: step uv lock --check has been added to GitHub Actions.

• Added test coverage control: pytest now crashes with an error if coverage is below 70% (--cov-fail-under).