feat: detect recurring subscription charges

[Robertg761]

/claim #109

Summary

• Add a recurring-charge detection service that groups same-merchant/same-amount expenses, normalizes statement noise, infers daily/weekly/monthly/yearly cadence, and skips income or generated recurring rows.
• Expose GET /expenses/recurring/detect with bounded min_occurrences and lookback_days query parameters.
• Document the endpoint in README/OpenAPI and add regression coverage for candidate detection and generated-row exclusion.

Validation

• . .venv/bin/activate && PYTHONPATH=packages/backend pytest packages/backend/tests/test_recurring_detection.py
• . .venv/bin/activate && black --check packages/backend/app/services/recurring_detection.py packages/backend/app/routes/expenses.py packages/backend/tests/test_expenses.py packages/backend/tests/test_recurring_detection.py
• . .venv/bin/activate && flake8 packages/backend/app/services/recurring_detection.py packages/backend/app/routes/expenses.py packages/backend/tests/test_expenses.py packages/backend/tests/test_recurring_detection.py
• git diff --check
• sh ./scripts/test-backend.sh tests/test_expenses.py could not run locally because Docker is not available (Docker is required to run backend tests.).