Plan 03 Summary: Adopt before_all in DB-Heavy Test Files

Tasks Completed

Task 1: Convert sources_index_metrics_test.rb to setup_once (17 read-only tests)
Task 2: Convert 3 single-test files to setup_once for consistency
Task 3: Verify all converted files individually (PARALLEL_WORKERS=1)
Task 4: Full suite verification (1033 tests, 0 failures) and lint (0 offenses)

test/lib/source_monitor/analytics/sources_index_metrics_test.rb (modified)
test/lib/source_monitor/analytics/source_activity_rates_test.rb (modified)
test/lib/source_monitor/analytics/source_fetch_interval_distribution_test.rb (modified)
test/lib/source_monitor/dashboard/upcoming_fetch_schedule_test.rb (modified)

Converted sources_index_metrics_test.rb from per-test setup to setup_once for shared fixture creation (3 sources + 3 items), following the reference pattern from query_test.rb (store IDs in setup_once, re-find records in per-test setup)
Kept travel_to/travel_back in regular setup/teardown for thread-local time safety
Converted 3 single-test files to setup_once for clean_source_monitor_tables! (functionally identical for single-test classes but normalizes the pattern)
setup_once usage increased from 1 file to 5 files across the test suite