Database indexing optimization for financial queries

[mendarb]

Summary

• Adds single-column and composite database indexes to all tables based on analysis of query patterns across expenses, dashboard, bills, reminders, and insights routes
• Targets the highest-impact queries: expense listing/filtering by date and category, dashboard aggregations by user+type+month, recurring expense dedup, bill listing by active+due date, and reminder processing
• Includes an idempotent SQL migration script (002_add_indexes.sql) and 16 tests validating index declarations at both SQLAlchemy metadata and database engine levels

Key indexes added

Table	Index	Columns	Rationale
expenses	ix_expenses_user_id_spent_at	user_id, spent_at	Expense listing with date range filter
expenses	ix_expenses_user_id_category_id	user_id, category_id	Category filter on expense list
expenses	ix_expenses_user_id_type_spent_at	user_id, expense_type, spent_at	Dashboard SUM aggregations
expenses	ix_expenses_user_id_recurring_spent_at	user_id, source_recurring_id, spent_at	Recurring expense dedup
categories	ix_categories_user_id_name	user_id, name (unique)	Category uniqueness enforcement
bills	ix_bills_user_id_active_due	user_id, active, next_due_date	Active bills sorted by due date
reminders	ix_reminders_user_id_sent_send_at	user_id, sent, send_at	Unsent reminder processing
reminders	ix_reminders_user_id_bill_id_channel_send_at	user_id, bill_id, channel, send_at	Bill reminder dedup

Closes #128

Test plan

• 16 new tests in test_db_indexes.py all passing
• Verify existing test suite still passes (pre-existing Redis connection failures unrelated)
• Run migration on staging PostgreSQL database
• Monitor query performance before/after with EXPLAIN ANALYZE on key queries

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