feat: database indexing optimization for financial queries

[alexchenai]

Summary

/claim #128

Adds 8 composite database indexes covering all high-frequency query patterns.

Indexes Added

• expenses: (user_id, spent_at) — date-range queries
• expenses: (user_id, category_id) — category breakdowns
• expenses: (user_id, amount) — amount range search
• bills: (user_id, next_due_date) — upcoming/overdue bills
• bills: (user_id, active) — active bills listing
• recurring_expenses: (user_id, active)
• reminders: (user_id, sent, send_at) — pending reminder dispatch
• categories: (user_id, name) — category lookups

Features

• Idempotent migration runner (IF NOT EXISTS)
• GET /insights/db/indexes — health check (admin only)
• POST /insights/db/indexes/migrate — apply migration (admin only)
• 10 unit tests