fix: move oauth flow intent literal into proto and align oauth state typing

[mplemay]

Summary

• Moved OAuthFlowIntent into belgie_proto and re-exported it through the existing public proto surface.
• Updated OAuthStateMixin to use Mapped[OAuthFlowIntent] and Mapped[JSONValue] so it matches OAuthStateProtocol.
• Kept the SQLAlchemy column definitions unchanged while fixing the type-level contract that blocked BelgieAdapter(..., oauth_state=OAuthState).

Testing

• uv run pytest packages/belgie-alchemy/src/belgie_alchemy/__tests__/unit/core/adapter/test_adapter.py passed.
• uv run ty check passed for an isolated BelgieAdapter(..., oauth_state=OAuthState) constructor check.
• uv run ruff check passed for the edited proto, oauth, and alchemy files.