Fix review issues: partial index, middleware scope, add timing tests

- Change idx_financing_plans_active to partial index (WHERE active = true)
  to avoid useless B-tree index on low-cardinality boolean column
- Scope timingMiddleware to /api routes only (was globally registered,
  which caused noise from static asset requests)
- Add unit tests for timing middleware (5 tests covering: next() call,
  timing log format, SLOW warning threshold, fast-request path, status code)

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

db/migrations/005_performance_indexes.sql

@@ -4,4 +4,4 @@ CREATE INDEX IF NOT EXISTS idx_paycheck_bills_paycheck_id ON paycheck_bills(payc
 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);
+CREATE INDEX IF NOT EXISTS idx_financing_plans_active ON financing_plans(active) WHERE active = true;