Add sqlx flatten support

[Vestibule]

Summary

• add flatten column expansion + bindings for CRUD
• add FlattenFields derive + Option support
• add flatten tests (no prefix/auto/custom)
• add docs for flatten usage

Testing

• cargo test -p atmosphere --features postgres --test integration -- flatten

Implementation choices

• Static expansion: flatten columns are generated at compile time (no runtime reflection).
• FlattenedColumn: concrete SQL column representation for nested fields, used by SQL generation + Bind.
• bind_field_optional: clean Option support (binds NULL when the nested value is absent).
• SELECT aliasing: contact_email AS email so sqlx::FromRow can rebuild the nested struct without prefix
  knowledge.
• Compile-time validation: assert_unique_table_columns::() checks name collisions including prefixed
  flatten columns.
• Direct recursion guard: blocks obvious self-recursive flatten types (including Option).
• sqlx compatibility: injects #[sqlx(flatten)] for FromRow.

Architecture

Resolves #42