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.

[LucasQR]

The codebase uses UUID v7 (uuid7), not UUID v4. Python's built-in uuid module only provides uuid1, uuid3, uuid4, and uuid5 — it does not support UUID v7.

UUID v7 (RFC 9562) is specifically chosen here because it's time-ordered, which matters for database performance. Random UUID v4 values scatter inserts across B-tree indexes, causing page splits and bloat in PostgreSQL. UUID v7 embeds a timestamp in the most significant bits, so values are generated in chronological rder and append to the end of the index — similar to auto-incrementing integers but without sacrificing global uniqueness.

The uuid6 package is a small, focused library that fills this gap in the stdlib. It's not a redundant wrapper around uuid it provides functionality that doesn't exist there yet.