feat(rust): Postgres schema object folders (functions, indexes, triggers, …) #1977

Parent

#1848

Goal

Flutter-parity Postgres object tree under each schema: Tables, Views, Materialized views, Functions, Sequences groups, plus Indexes / Triggers / Types listings.

Done when

• Expanding a schema shows those folders (not a flat table/view dump)
• Functions, sequences, indexes, triggers, types list via db_list_children
• Unit/live tests cover shop schema folders

How to work

• Branch: issue/<n>-slug from epic/tauri-migration
• One PR → base epic/tauri-migration
• Also include typed SqlGuardRule on ConnectionError + TS ConnectionError union (prep for feat(rust): dangerous DML confirmation gate #1885)