Known backend limitations

Both limitations this section used to list have been fixed, and the section is kept only to say so — they were quoted often enough that their absence is worth stating.

• SQLite — adding foreign keys. Fixed. create_constraints_sql_with_dialect now returns nothing when the dialect inlines FKs, and inline_fk_clauses emits them inside CREATE TABLE. The earlier workaround skipped FKs on SQLite entirely, which silently dropped referential integrity rather than merely failing.
• SQLite — apply_all_pool and framework models. Fixed. It walks bootstrap_models() and the DDL is emitted per dialect, so the framework's own tables create cleanly on SQLite. Verified by running it against sqlite::memory: with tenancy and batteries on — all nineteen managed framework models (rustango_orgs, rustango_operators, rustango_audit_log, …) applied without error.