docs: document async_db_url and reflex[db] extra (ENG-8371)

[adhami3310]

What

Fixes ENG-8371.

Two gaps in the database docs:

1. async_db_url was undocumented. rx.asession() requires a separate async_db_url (with an async driver) in rxconfig.py, and raises ValueError("No async database url configured") when it is unset — but nothing in the docs mentioned this.
2. The reflex[db] extra was undocumented. The ORM dependencies (SQLAlchemy, SQLModel, Alembic) are an optional extra, installed via pip install "reflex[db]".

Changes

• docs/database/overview.md: new Installation section covering pip install "reflex[db]".
• docs/database/queries.md: new Configuring the Async Database URL subsection with a config example (sqlite+aiosqlite), the async-driver install step, and a note on the PostgreSQL (asyncpg) variant.

Docs-only change.

[linear-code]

ENG-8371

[greptile-apps]

Greptile Summary

This docs-only PR fills two gaps in the database documentation: the undocumented reflex[db] optional extra and the required async_db_url configuration for rx.asession.

• docs/database/overview.md: adds an Installation section documenting pip install "reflex[db]" to install SQLModel and Alembic as an optional extra.
• docs/database/queries.md: adds a Configuring the Async Database URL subsection with a complete rxconfig.py example, per-driver install steps (aiosqlite for SQLite, psycopg3 for PostgreSQL), and a note that async drivers are not bundled in the reflex[db] extra. Previously-reviewed concerns (dangling "as shown above", incorrect dependency listing) are resolved in this revision.

Confidence Score: 5/5

Docs-only change with no runtime code modified; safe to merge.

Both additions are accurate: the reflex[db] extra does install SQLModel and Alembic, async_db_url is genuinely required for rx.asession, and the psycopg3 claim that a single postgresql+psycopg:// URL works for both sync and async engines is correct per SQLAlchemy 2.x + psycopg3 documentation. Previously flagged issues from earlier review rounds are resolved in this revision.

No files require special attention.

Important Files Changed

Filename	Overview
docs/database/overview.md	Adds a new "Installation" section documenting the reflex[db] optional extra (SQLModel + Alembic), placed appropriately between the NoSQL callout and the Connecting section.
docs/database/queries.md	Adds a "Configuring the Async Database URL" subsection before the existing rx.asession usage examples, covering the required async_db_url config, aiosqlite install for SQLite, and psycopg3 for PostgreSQL. Previously-flagged issues (dangling "as shown above", asyncpg install step) are resolved.

_{Reviews (3): Last reviewed commit: "docs: tighten async database prose" | Re-trigger Greptile}

[chatgpt-codex-connector]

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[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing claude/angry-shtern-7f3f2b (899ce98) with main (4c63f00)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩