feat: index usage_events into revenue_ledger

[MerlinTheWhiz]

Closes #311

Key changes made:

• Added revenue ledger background indexer with cursor batching + idempotent inserts
• src/repositories/usageEventsRepository.pg.ts: added cursor/index repository methods
• src/services/revenueLedgerIndexer.ts: added background indexer job
• src/index.ts: wired job startup + graceful shutdown
• src/services/usageStore.ts: resolved developer ownership from apis
• src/config/env.ts: added env config/docs
• src/config/index.ts: added indexer config support
• .env.example: added new env variables
• README.md: documented setup/config changes
• src/repositories/usageEventsRepository.pg.test.ts: added repository tests
• src/services/revenueLedgerIndexer.test.ts: added indexer job tests
• src/config/env.test.ts: added env config tests

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[greatest0fallt1me]

Merged via direct push to main (admin)