feat: Database indexing optimization for financial queries (#128)

[lalalic]

Summary

Implements comprehensive database indexing optimization for financial queries, addressing issue #128. Adds 20+ targeted indexes across all application tables with a performance monitoring API.

What it does

Index Strategy

Adds optimized indexes following the "equality → range → sort" principle for maximum query efficiency:

Expenses (highest volume):

• idx_expenses_user_date — Primary listing (every page load)
• idx_expenses_user_category_date — Category reports
• idx_expenses_user_date_amount — Monthly totals and SUM queries
• idx_expenses_notes_trgm — Full-text search (GIN trigram)
• idx_expenses_user_type — Income vs expense filtering

Bills & Reminders:

• idx_bills_user_due — Upcoming bills dashboard (partial: active only)
• idx_reminders_pending — Cron job unsent reminders (partial)

All other tables — Categories, users, ad impressions, audit logs, subscriptions

Performance Monitoring

Method	Endpoint	Description
GET	/admin/db/indexes	List all indexes across tables
GET	/admin/db/coverage	Query pattern coverage analysis
GET	/admin/db/statistics	Table row counts
POST	/admin/db/benchmark	Run query benchmarks with timing

Files Changed

File	Description
app/db/026_indexing_optimization.sql	20+ PostgreSQL index definitions with partial indexes and trigram
app/models.py	SQLAlchemy model-level Index() declarations on 6 models
app/services/indexing.py	Index verification, coverage analysis, benchmarks, statistics
app/routes/indexing.py	Admin API endpoints
app/routes/__init__.py	Blueprint registration
tests/conftest.py	fakeredis test isolation
tests/test_indexing.py	27 comprehensive tests
docs/db-indexing-optimization.md	Full documentation

Testing

27 passed in 0.74s

• 6 model index definition verification tests
• 3 index verification service tests
• 5 prefix utility tests
• 4 coverage analysis tests
• 2 benchmark tests
• 2 table statistics tests
• 5 API endpoint tests (including auth)

Index Coverage Report Example

{
  "total_patterns": 9,
  "covered": 8,
  "missing": 1,
  "coverage_percentage": 88.9
}

Closes #128