fix: use relative imports in models.py for Docker compatibility

[AndyBodnar]

Summary

• Changed absolute imports to relative imports in aperisolve/models.py
• Fixes module import errors when running in Docker with the Postgres container
• The absolute imports caused circular import issues in the containerized environment

Test plan

• Build the Docker image with docker-compose build
• Start the containers with docker-compose up
• Verify the Postgres container starts without module import errors
• Test that AperiSolve functions correctly with the database

Fixes #168

[Zeecka]

As mentioned in issue:

Also, using absolute imports is recommended :

Clear and explicit: you always know where things come from.
Stable when refactoring: moving files doesn’t silently break imports.
Works everywhere: scripts, tests, REPL, CLI entrypoints, workers, etc.
PEP 328 / PEP 8 recommended for top-level packages.
Plays nicely with tooling (IDE navigation, static analysis, mypy, pytest).