feat(WHE-29): add v1 accounting schema migration and seed fixtures

docs/specs/HOUSEBOT-012-accounting-schema.md

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+# HOUSEBOT-012: V1 Accounting Schema
+
+## Summary
+
+Define and migrate a production-ready accounting schema for household rent/utilities/purchases and settlement snapshots.
+
+## Goals
+
+- Support all v1 finance flows in database form.
+- Enforce idempotency constraints for Telegram message ingestion.
+- Store all monetary values in minor integer units.
+- Provide local seed fixtures for fast end-to-end development.
+
+## Non-goals
+
+- Bot command implementation.
+- Settlement algorithm implementation.
+
+## Scope
+
+- In: Drizzle schema + generated SQL migration + seed script + runbook update.
+- Out: adapter repositories and service handlers.
+
+## Interfaces and Contracts
+
+Tables covered:
+
+- `households`
+- `members`
+- `billing_cycles`
+- `rent_rules`
+- `utility_bills`
+- `presence_overrides`
+- `purchase_entries`
+- `processed_bot_messages`
+- `settlements`
+- `settlement_lines`
+
+## Domain Rules
+
+- Minor-unit integer money only (`bigint` columns)
+- Deterministic one-cycle settlement snapshot (`settlements.cycle_id` unique)
+- Message idempotency enforced via unique source message keys
+
+## Data Model Changes
+
+- Add new finance tables and indexes for read/write paths.
+- Add unique indexes for idempotency and period uniqueness.
+- Preserve existing household/member table shape.
+
+## Security and Privacy
+
+- Store internal member IDs instead of personal attributes in finance tables.
+- Keep raw purchase text for audit/parser debugging only.
+
+## Observability
+
+- `processed_bot_messages` and settlement metadata provide traceability.
+
+## Edge Cases and Failure Modes
+
+- Duplicate message ingestion attempts.
+- Missing active cycle when writing purchases/bills.
+- Partial fixture seed runs.
+
+## Test Plan
+
+- Generate and check migration metadata.
+- Typecheck db package.
+- Execute seed script on migrated database.
+
+## Acceptance Criteria
+
+- [ ] `bun run db:generate` produces migration SQL for schema updates.
+- [ ] `bun run db:check` passes.
+- [ ] Schema supports v1 rent/utilities/purchase/settlement workflows.
+- [ ] Idempotency indexes exist for Telegram ingestion.
+- [ ] Migration runbook includes rollback notes.
+
+## Rollout Plan
+
+- Apply migration in dev, run seed, validate queryability.
+- Promote migration through CI/CD pipeline before production cutover.