docs: add roadmap, ADRs, and initial HOUSEBOT specs

docs/specs/HOUSEBOT-002-finance-domain-model.md

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+# HOUSEBOT-002: Finance Domain Model
+
+## Summary
+Define domain entities and invariants for rent, utilities, shared purchases, and monthly settlements.
+
+## Goals
+- Create a deterministic model for monthly household accounting.
+- Encode money-safe arithmetic with integer minor units.
+- Support equal split by default with optional day-based utility override.
+
+## Non-goals
+- Telegram command handlers.
+- Mini app rendering.
+
+## Scope
+- In: domain entities, use-case contracts, and validation rules.
+- Out: persistence adapter implementation.
+
+## Interfaces and Contracts
+- `Money` value object (`currency: GEL`, `amountMinor: bigint | number`).
+- `SettlementInput` contract with members, rent, utilities, purchases, overrides.
+- `SettlementResult` contract with per-member due and explanation lines.
+
+## Domain Rules
+- Rent is fixed per cycle and split equally among active members.
+- Utilities split equally unless per-member day override is provided.
+- Shared purchases reduce payer due amount and distribute cost across members.
+- No floating-point operations.
+
+## Data Model Changes
+- Define required table contracts (implemented in later DB ticket):
+  - `billing_cycles`
+  - `rent_rules`
+  - `utility_bills`
+  - `presence_overrides`
+  - `purchase_entries`
+  - `settlements`, `settlement_lines`
+
+## Security and Privacy
+- Do not store unnecessary personal data.
+- Use internal IDs for members in calculations.
+
+## Observability
+- Structured calculation logs (input hash, cycle id, result totals).
+- Error event on invalid settlement state.
+
+## Edge Cases and Failure Modes
+- Member count is zero.
+- Utility day overrides sum to zero.
+- Negative amounts from malformed inputs.
+- Duplicate purchase entries.
+
+## Test Plan
+- Unit:
+  - money arithmetic and normalization
+  - equal split and day-weighted split cases
+  - purchase offsets and reconciliation checks
+- Integration: not in this ticket.
+- E2E: not in this ticket.
+
+## Acceptance Criteria
+- [ ] Value objects implemented and tested.
+- [ ] Settlement input/output contracts defined.
+- [ ] Deterministic settlement math covered by tests.
+- [ ] Edge cases produce explicit domain errors.
+
+## Rollout Plan
+- Merge as dependency for settlement engine and bot handlers.