Fix category deletion and stale group tally after expense changes

[LouisBenjamin]

Two bugs, reported together

1. Categories couldn't be deleted

CategoriesRepository.deleteCategory ran a plain delete from categories where id = ?. That has two failure modes, both of which look like "delete does nothing":

• Silent zero-row delete when RLS hides the target — a global default (group_id is null), or a category owned by another group. PostgREST returns 204, no error, the list refetches, the row is still there.
• Foreign-key violation anywhere the 0012 on delete set null cascade isn't in effect (an environment still on <= 0011) — expenses / recurring_expenses / merchant_rules still point at the row.

Fix: new delete_category(id) SECURITY DEFINER RPC (supabase/migrations/0013_delete_category_rpc.sql), mirroring delete_expense / update_expense / update_group_photo. It:

• authorizes the caller as a member of the category's group,
• raises a real error for "not found" / "default category" / "not authorized",
• nulls out every expenses / recurring_expenses / merchant_rules reference explicitly, so it works regardless of the FK action,
• deletes the row.

deleteCategory now calls the RPC.

2. Deleting an expense didn't fix the tally

The "you owe / you're owed" line on each groups-list card comes from groupSummariesProvider (the my_group_summaries view). Every expense mutation site — add, edit, delete, import — and the settle-up flow invalidated expensesProvider and balancesProvider but not groupSummariesProvider, which nothing else refreshes. Navigation is context.push, so GroupsListScreen stays mounted underneath and reuses its cached value: the tally stayed stale until a manual pull-to-refresh.

Fix: invalidateGroupMoney(ref, groupId) in groups_provider.dart invalidates all three, and replaces the ad-hoc pairs at every expense / settlement mutation point (group_detail_screen.dart, balances_tab.dart).

Testing

• flutter analyze clean, flutter test — 180 existing + 1 new pass.
• New test/features/groups/group_money_invalidation_test.dart asserts invalidateGroupMoney refetches the groups-list summary, not just the in-group providers.
• delete_category RPC still needs supabase db push (or running 0013 in the SQL editor) against your project.

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