feat: expense tracker Python parity (Wave 10)
New `expenses_handlers.py`:
- parse_expense(phrase) -> (amount, category) | None — pure fn, fully
unit-tested. Handles "потратил 500 на еду", comma decimals "12,50",
EN "spent 12.5 on coffee", default category "разное".
- do_expenses_log: append entry to expenses.json (atomic write-then-rename).
- do_expenses_today: sum today's entries (local-day boundary).
- do_expenses_week: rolling 7-day total + count.
- do_expenses_breakdown: per-category, top-5 + tail collapse.
extensions.py / main.py / commands.yaml: 4 new dispatch entries.
.gitignore: + expenses.json.
tests/test_expenses_handlers.py: 8 unit tests covering parse, log,
today/week math, breakdown ordering + tail collapse.
Tests: 96 → 104 (+8). Total Python coverage now mirrors Rust pack
behaviour for personal finance.
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