fix(oauth): increase access_token and refresh_token length to 4096

[Sohail342]

When using OAuth2 providers such as Authentik with RS256-signed access tokens, the access_token (and sometimes refresh_token) exceeds 1024 characters. This results in a StringDataRightTruncation error in PostgreSQL:

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

Changes

• Increased access_token and refresh_token field lengths from 1024 to 4096.
• Verified behavior against PostgreSQL using long RS256 tokens.
• All tests pass with 100% coverage using hatch run test.

Configuration

• Python: 3.13
• FastAPI: 0.115.12
• fastapi-users-db-sqlalchemy: latest main

This should allow broader compatibility with modern OAuth2 providers.