deps: remove uuid and uuid6 packages

[PeakPy]

This PR addresses the "UUID usage" part of issue #248 by removing the direct dependency on external uuid / uuid6 packages and relying on Python's built-in uuid module instead.

Changes:

• Remove uuid and uuid6 from project dependencies in pyproject.toml.
• Replace uuid7 usage with uuid.uuid4() in:
  • src/scripts/create_first_superuser.py
  • src/app/models/user.py
  • src/app/models/post.py
  • src/app/core/schemas.py
  • src/app/core/db/models.py
• Update tests to use uuid.uuid4() in tests/conftest.py.

Tests:

• uv run pytest
• uv run ruff check src
• uv run mypy src

This is a follow-up to issue #248.