fix: SQLAlchemyAdapter degrades gracefully when async DB driver is missing (1.2.1)

[svalench]

Summary

Hotfix for v1.2.0 regression: from fastapi_viewsets import BaseViewset (or any code path that initialises the default SQLAlchemyAdapter) crashes at startup with ModuleNotFoundError: No module named 'aiosqlite' when the async DB-API driver is not installed — even for purely synchronous setups that never use async sessions.

Root cause

In #3, ORMFactory.get_adapter_from_env started always passing an explicit async_database_url (e.g. sqlite+aiosqlite:///...) to SQLAlchemyAdapter. The adapter's existing try/except around create_async_engine() only covered the auto-converted-URL branch — the explicit-URL branch raised ModuleNotFoundError directly from __init__.

Stack trace from a user with no aiosqlite installed:

File "fastapi_viewsets/orm/sqlalchemy_adapter.py", line 74, in __init__
    self.async_engine = create_async_engine(async_database_url, echo=False)
...
File "sqlalchemy/dialects/sqlite/aiosqlite.py", line 449, in import_dbapi
    __import__("aiosqlite"), __import__("sqlite3")
ModuleNotFoundError: No module named 'aiosqlite'

Fix

• Both async-engine creation branches are now best-effort. On any failure (missing driver, no async dialect for the URL, etc.) async_engine and AsyncSessionLocal fall back to None, and get_async_session() raises a helpful RuntimeError lazily with installation hints. Sync usage works without any extra dependency.
• Bumped version to 1.2.1.
• Switched pyproject.toml packaging to [tool.setuptools.packages.find] so future subpackages are picked up automatically.
• Added regression tests for both URL paths (explicit async_database_url and auto-converted URL) when the async driver import fails.

Verification

• Repro: created adapter without aiosqlite → adapter constructs cleanly, sync session works, get_async_session() raises a friendly RuntimeError.
• Built wheel and confirmed fastapi_viewsets/orm/* is included.
• Test suite: 208 passed, 39 skipped (skips are pre-existing tortoise/peewee env-dependent tests).

Release notes

See RELEASE_1.2.1.md and updated RELEASE_NOTES.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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