feat: Add expense tracking

[QueryPlanner]

Priority: Medium

Level of Effort: Large (5-8 days)

Summary

Add expense tracking capability following the same pattern as calorie and workout tracking. Users can log expenses with categories, amounts, and notes; the agent can provide summaries and spending insights.

Scope

• New module: src/blacki/expenses/
• Storage: PostgresExpenseStorage (table: expenses, extends PostgresStorage base)
• Data model:
  • amount (Decimal, required — supports precision for currency)
  • category (str, required — e.g., "food", "transport", "entertainment", "utilities", "shopping")
  • description (str, optional)
  • currency (str, default "INR", configurable)
  • spent_at (datetime UTC, auto-set to current time if not provided)
  • spent_date (date local, for daily aggregation)
• Tools:
  1. log_expense — record an expense with amount, category, and optional description
  2. get_expense_summary — daily summary or multi-day range with category breakdown and total
  3. edit_expense — update an existing expense entry
  4. delete_expense — remove an expense entry
  5. set_expense_budget — set a daily/weekly/monthly budget goal (stored in preferences)
  6. list_expenses — list recent expenses with optional date range and category filter
  7. get_spending_insights — compare spending to budget, highlight top categories, identify trends
• Preferences: Budget goal stored in PostgresPreferencesStorage under keys like expense_budget_monthly, expense_budget_weekly
• Date support: Use existing dateparser integration from src/blacki/utils/dates.py for natural language dates
• Pattern: Follow existing storage + tools patterns from calories/ and workouts/

Sources

• Calorie storage: src/blacki/calories/storage.py (model pattern)
• Calorie tools: src/blacki/calories/tools.py (tool pattern, summary aggregation)
• Workout storage: src/blacki/workouts/storage.py
• Storage base class: src/blacki/storage/base.py
• Preferences storage: src/blacki/utils/preferences.py
• Date utilities: src/blacki/utils/dates.py
• Tool registry: src/blacki/registry.py
• DI container: src/blacki/container.py
• Agent prompt: src/blacki/prompt.py
• AGENTS.md tool design principles

Passing Criteria

• uv run ruff check passes with no errors
• uv run ruff format passes with no changes needed
• uv run mypy . passes with no errors
• uv run pytest tests/expenses/ --cov=src/blacki/expenses passes with 100% coverage on expenses module
• uv run pytest full suite passes
• log_expense correctly stores expense entry scoped to user_id with all fields
• get_expense_summary returns correct daily totals and category breakdowns
• get_expense_summary supports multi-day ranges with aggregation
• edit_expense updates fields correctly and returns updated record
• delete_expense removes the record and returns confirmation
• set_expense_budget persists budget preference and returns confirmation
• get_spending_insights compares spending to budget and highlights top categories
• list_expenses supports filtering by date range and category
• Storage is properly initialized via AppContainer and registry.py
• All tools support natural language dates via existing dateparser integration