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+# Mini App Admin Billing Controls
+
+## Summary
+
+Allow household admins to manage billing-cycle configuration from the mini app: rent amount, reminder timing, utility timing, utility categories, and admin promotion.
+
+## Goals
+
+- Let admins control billing settings without bot commands or Terraform edits.
+- Persist household-level billing preferences in the database.
+- Support configurable utility categories per household.
+- Let admins promote other household members to admin from the mini app.
+- Provide a stable API contract for future reminder and finance UI.
+
+## Non-goals
+
+- Full accounting redesign.
+- Per-member notification preferences.
+- Utility OCR/import integrations.
+- Member removal/deactivation in this slice.
+
+## Scope
+
+- In: household billing settings persistence, utility category management, admin promotion, mini app admin endpoints/UI.
+- Out: historical category migration tools, reminder delivery redesign, presence/day-based utility UI.
+
+## Interfaces and Contracts
+
+- Mini app admin endpoints:
+  - `POST /api/miniapp/admin/settings`
+  - `POST /api/miniapp/admin/settings/update`
+  - `POST /api/miniapp/admin/utility-categories/upsert`
+  - `POST /api/miniapp/admin/members/promote`
+- Admin-only mini app settings screen sections:
+  - Rent
+  - Utility categories
+  - Reminder timing
+  - Admins
+
+## Domain Rules
+
+- Only active household admins may update household billing settings.
+- Billing timing is household-scoped, not user-scoped.
+- Utility categories are household-scoped and deterministic in display order.
+- Promotion can only target active household members of the same household.
+- At least one admin must remain in the household.
+- Money remains stored in minor units only.
+
+## Data Model Changes
+
+- Add `household_billing_settings` one-to-one table:
+  - `household_id`
+  - `rent_currency`
+  - `rent_amount_minor` nullable until configured
+  - `rent_due_day`
+  - `rent_warning_day`
+  - `utilities_due_day`
+  - `utilities_reminder_day`
+  - `timezone`
+  - timestamps
+- Add `household_utility_categories` table:
+  - `household_id`
+  - `slug`
+  - `name`
+  - `sort_order`
+  - `is_active`
+  - timestamps
+- Seed default categories for new and existing households:
+  - `internet`
+  - `gas_water`
+  - `cleaning`
+  - `electricity`
+- Add nullable `utility_category_id` to `utility_bills` in a later follow-up if needed for entry-level classification.
+
+## Security and Privacy
+
+- All endpoints require authenticated mini app sessions.
+- All write endpoints require admin authorization from persisted membership.
+- Promote-admin actions should be logged with actor and target IDs.
+
+## Observability
+
+- Structured logs for settings updates, category writes, and admin promotion.
+- Include `householdId`, `actorMemberId`, `targetMemberId`, and changed fields.
+
+## Edge Cases and Failure Modes
+
+- Invalid day values outside 1..31.
+- Reminder day later than due day.
+- Duplicate category slug in one household.
+- Promote-admin target missing or already admin.
+- Missing settings record for an existing household should auto-bootstrap defaults.
+
+## Test Plan
+
+- Unit:
+  - settings validation rules
+  - category upsert ordering and duplicate handling
+  - admin promotion authorization
+- Integration:
+  - DB adapter round-trips for settings and categories
+- E2E:
+  - admin changes settings in mini app and sees persisted values on reload
+
+## Acceptance Criteria
+
+- [ ] Admin can view and update household billing settings in the mini app.
+- [ ] Admin can manage household utility categories in the mini app.
+- [ ] Admin can promote an active member to admin in the mini app.
+- [ ] Reminder timing values are persisted per household.
+- [ ] Existing households auto-bootstrap sensible defaults.
+
+## Rollout Plan
+
+- Add schema and repository support first.
+- Ship read-only settings payload next.
+- Enable admin writes after UI is wired.
+- Keep bot command paths working during rollout.