Upgrade to Poetry 2.x, PEP 621, and Python >=3.10

[sandhose]

Summary

• Migrate pyproject.toml from [tool.poetry] to PEP 621 [project] table
• Move dev dependencies to [dependency-groups] (PEP 735)
• Bump minimum Python from 3.8 to 3.10 (3.8 and 3.9 are EOL)
• Upgrade CI Poetry action from v1 to v2 (Poetry 2.2.1)
• Update Dockerfile base image to Python 3.11, add libffi-dev and poetry-plugin-export
• Upgrade black from 21.6b0 to >=23.1.0 (old version incompatible with modern click)
• Drop typing_extensions dependency, use typing stdlib equivalents
• Fix Python 3.14 test compat (asyncio.Future() → AsyncMock in test_msisdn)

Test plan

• poetry install works with Poetry 2.2.1
• CI green: linting, type checking, unit tests, integration tests (3.10 + 3.11), Docker build

Part 0 of 9 in the modernisation series.