fix(cost): wire MeteringService lifecycle for persistent cost tracking (#3310)

[OneStepAt4time]

Summary

Fixes #3310

The MeteringService was instantiated but its load()/save() lifecycle methods were never called, and the JsonlWatcher only processes file changes — not existing content. This meant cost data was always zero after every server restart.

Changes

1. Startup load: Call metering.load() after instantiation to restore persisted records from previous runs
2. JSONL backfill: After setting up JSONL watches, replay all existing JSONL data from offset 0 for sessions that already have data (monitorOffset > 0). This backfills metering for sessions completed before the current server start
3. Shutdown save: Add metering.save() to the graceful shutdown handler (alongside existing metrics.save())
4. Periodic save: Add meteringSaveInterval (5-minute cadence, same as metrics) to persist data periodically

Verification

npx tsc --noEmit  ✅
npm run build     ✅
npm test          ✅ 4147 passed, 2 skipped (1 pre-existing flaky timeout)

Notes

• The ACP-specific part of bug: cost data is zero despite 140 sessions — BillingMeteringService not capturing ACP token data #3310 (event-mapper dead code) is not addressed here — that requires a separate change to wire ACP notification events to the metering service
• The backfill only processes sessions with monitorOffset > 0 to avoid re-processing sessions where no data has been read yet (those will be picked up by the watcher naturally)

[aegis-gh-agent]

✅ Approved — code review complete, pending CI.

Review summary:

• Lifecycle wiring for MeteringService: load() at startup, save() on shutdown + periodic (5min)
• JSONL backfill for sessions with monitorOffset > 0 — correct, only replays sessions with existing data
• Error handling consistent: all load/save/backfill failures are non-critical (logged or silently caught)
• Follows existing metrics save pattern (same interval, same shutdown handler approach)
• No secrets, no security concerns
• Targets develop ✅

Minor note: No dedicated tests for the server.ts wiring, but this is lifecycle glue — the MeteringService itself has tests. Acceptable for this scope.

Will merge once CI goes green.