fix: increase OAuth token column length to 4096

[pctablet505]

Fixes fastapi-users/fastapi-users#1523

When an OAuth provider signs tokens with RS256, the resulting access and refresh tokens can easily exceed the 1024 characters we were allowing for access_token and refresh_token in SQLAlchemyBaseOAuthAccountTable. The reporter hit this with Authentik on PostgreSQL:

sqlalchemy.exc.DataError: (psycopg.errors.StringDataRightTruncation) value too long for type character varying(1024)

Another user confirmed the same failure on MariaDB:

sqlalchemy.exc.DataError: (asyncmy.errors.DataError) (1406, "Data too long for column 'access_token' at row 1")

In both cases the token cannot be persisted, so the login fails.

The fix increases both columns from String(length=1024) to String(length=4096), matching the length the reporter is already using as a workaround by overriding the column definitions in their own model. I also added a round-trip test that writes and reads back a 2048-character access token, so we don't accidentally truncate long tokens in the future.

Existing deployments will still need a manual Alembic migration (or equivalent) to widen already-created columns; SQLAlchemy won't resize them automatically.

[pctablet505]

Closing this draft — PR #30 already covers the same fix, and the repo is in maintenance mode (discussion #1543).