Allow ForeignKey(index=True) to emit a single-column index on the shadow column

[0x054]

FerroField(index=True) works on plain columns, but ForeignKey doesn't accept index. Postgres doesn't auto-index FK columns, and most multi-tenant schemas need indexes on org_id-style FK columns for both ordinary tenant queries and (in our case) RLS policy evaluation.

Proposed API

ForeignKey(
    related_name=...,
    on_delete=...,
    unique=False,
    index=False,        # <-- new
    nullable='infer',
)

When index=True, autogen emits CREATE INDEX on the shadow *_id column the same way FerroField(index=True) does for plain columns. unique=True already produces an implicit unique index, so the new flag is the non-unique variant.

Acceptance criteria

• ForeignKey(..., index=True) causes Alembic autogen to emit a non-unique index on the shadow *_id column.
• index=False (default) keeps current behavior.
• index=True + unique=True is allowed and emits both (or just the unique index, since unique implies index — implementer's call).
• Index name follows existing autogen convention.

Motivation

Same downstream project (Blueberry / RLS): every tenant-scoped table has an org_id FK that is queried on every list endpoint. We're hand-editing eight indexes into the migration today; would prefer to declare them on the model.