Prepare AirPostgres 0.2.0

[audreyfeldroy]

Summary

Prepare AirPostgres 0.2.0, the first release containing the actual PostgreSQL backend.

• bump the package from the 0.1.0 scaffold to 0.2.0
• document asyncpg CRUD, query, bulk, lifecycle, transaction, and schema support
• document migration from Air's former built-in PostgreSQL integration
• pin the draft's development source to the exact Air 0.49 release-preparation commit so clean CI checkouts can resolve the new backend contract

Validation

• just check — 126 passed on Python 3.14; format, Ruff, and ty clean
• just testall — 126 passed on Python 3.13 and 126 passed on Python 3.14
• just coverage — 92%
• just docs-build
• built sdist and wheel
• installed the Air 0.49 and AirPostgres 0.2 wheels together in a clean Python 3.13 environment and verified versions and public imports

Release sequencing

Air 0.49 must be published first. Before merging and releasing this PR, remove the temporary Git source override for Air and regenerate uv.lock against the published Air 0.49 package.

This PR prepares the release only; it does not tag or publish 0.2.0.