Add performance regression detection: indexes, timing middleware, benchmark script

Nightshift-Task: perf-regression
Nightshift-Ref: https://github.com/marcus/nightshift

db/migrations/005_performance_indexes.sql

@@ -0,0 +1,7 @@
+-- Performance indexes for high-traffic query patterns
+CREATE INDEX IF NOT EXISTS idx_paychecks_period ON paychecks(period_year, period_month);
+CREATE INDEX IF NOT EXISTS idx_paycheck_bills_paycheck_id ON paycheck_bills(paycheck_id);
+CREATE INDEX IF NOT EXISTS idx_actuals_paycheck_id ON actuals(paycheck_id);
+CREATE INDEX IF NOT EXISTS idx_one_time_expenses_paycheck_id ON one_time_expenses(paycheck_id);
+CREATE INDEX IF NOT EXISTS idx_financing_payments_plan_id ON financing_payments(plan_id);
+CREATE INDEX IF NOT EXISTS idx_financing_plans_active ON financing_plans(active);