Fix FK metadata loading

[m-tonon]

Summary

Fix PostgreSQL foreign key metadata loading by swapping the information_schema FK query for a pg_catalog one.

The old query sometimes failed to load certain foreign keys — depending on how PostgreSQL exposed things through information_schema. When that happened, the FK buttons just disappeared from the UI.

The new query reads directly from PostgreSQL system catalogs (pg_constraint, pg_attribute, pg_class, pg_namespace) and correctly handles:

• foreign key relationships
• composite foreign keys
• update/delete rules

Changes

• Replaced information_schema FK query with pg_catalog query
• Added integration test covering:
  • single-column foreign keys
  • composite foreign keys
  • cross-schema references
  • batch FK loading

Testing

Ran:

cargo test test_postgres_foreign_keys_via_pg_catalog -- --ignored

Also manually checked that FK buttons and navigation work in the editor UI.

Closes #244

[debba]

Thanks for this PR :)
I'll merge it now.