Improve typing for db["table"] usage

[RamiNoodle733]

Summary

• annotate Database.__getitem__ as returning Table for static type checkers
• keep existing runtime behavior for views by casting the view return path
• add typing-focused regression tests for the return annotation and view runtime behavior

Why

Fixes #638, where mypy/pyright report errors for common db["name"].create(...) and upsert(...) usage.

Validation

• uv run flake8 sqlite_utils/db.py tests/test_typing.py
• uv run mypy sqlite_utils tests
• uv run pytest tests/test_typing.py -q
• uv run pytest tests/test_create.py -k view -q