Prepare Air 0.49.0

[audreyfeldroy]

Summary

Prepare Air 0.49.0 for release and backport the complete backend-neutral work staged in Writerstead's vendored Air tree.

• make AirModel persistence backend-neutral through the AirModelBackend protocol
• preserve the AirDatabase convenience base and DatabaseNotConfigured error
• move PostgreSQL transport, SQL, schema, and lifecycle concerns out of Air core
• remove the bundled asyncpg extra and automatic DATABASE_URL/app.db behavior
• port AirForm.render_csrf() for custom form layouts
• include the post-0.48.1 HTML/Jinja improvements merged in Docs improvements #1162
• document migration to AirPostgres and other external backends
• add AirModel, field/model API, Railway, and WebAssembly documentation to navigation and LLM exports
• collect the full 0.49.0 release notes, including air check, WebAssembly support, form/HTML improvements, and fixes merged since 0.48.1

Release impact

This is a breaking migration for applications that use Air's former built-in PostgreSQL integration. Model definitions and public CRUD calls remain stable, but applications must install and configure AirPostgres explicitly.

AirPostgres 0.2.0 is prepared as the companion backend release. AirD1 has also been extracted, reviewed, and validated against this Air release line.

Validation

• rebased onto current main after Docs improvements #1162
• just qa
• just test — 705 passed
• strict MkDocs build
• verified generated field/model API pages, AirModel and WebAssembly pages, navigation, and llms-full.txt
• built the exact committed sdist and wheel from a clean archive
• installed the wheel in a clean Python 3.13 environment and verified version 0.49.0 and public backend imports
• AirPostgres — 126 tests on Python 3.13 and 3.14, 92% coverage, docs build, CI green
• AirD1 — 14 tests, 95% coverage, CI green
• Writerstead vendored Air matches every shared PR file byte-for-byte
• Writerstead — 17 tests passed; Worker bundle and Wrangler dry-run passed; workflow lint passed

Publication

This PR prepares the release only. Pushing tag v0.49.0 will trigger the trusted PyPI publication workflow, which requires approval in the protected pypi environment.