Fix db_default on FK/O2O dropped during _init_relations

[mixartemev]

Description

_init_relations builds the implicit <fk>_id key field by copying the related model's PK field and then patching a small set of attributes from the FK object:

for attr in ("index", "default", "null", "generated", "description"):
    setattr(key_fk_object, attr, getattr(fk_object, attr))

db_default is missing from this list. The schema editor then iterates fields_db_projection (which holds the key field, not the FK relation field) and inspects key_field.has_db_default(). It returns False, so CREATE TABLE for the owning model is emitted without a DEFAULT clause on the FK column — even when db_default=... is set on the ForeignKeyField / OneToOneField and faithfully recorded in the generated migration.

This fix adds "db_default" to the attribute-copy tuple. Same path covers OneToOneField since both go through init_fk_o2o_field.

Motivation and Context

Fixes #2199

PR #2129 fixed the same omission for non-relation fields (the schema editor now emits DEFAULT for plain columns). This is the FK twin of that bug, living one layer up in Apps._init_relations — the schema editor never gets a chance to see the default because it isn't on the key field.

How Has This Been Tested?

Added test_create_model_includes_db_default_on_fk in tests/migrations/test_schema_editor_sql.py:

1. Declares a target model Dc and a model App with dc = ForeignKeyField("models.Dc", db_default=2).
2. Runs init_apps(Dc, App) (which exercises StateApps._init_relations — the fixed path).
3. Calls create_model(App) via the schema editor.
4. Asserts the SQL contains "dc_id" and DEFAULT 2.

The test fails on develop HEAD without the fix (verified locally), and passes with it. All 298 existing migration tests continue to pass.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.