fix(billing): fix critical bugs in consumeFromOrderedGrants

Fix three critical bugs in credit consumption:
1. Stale balance check - grant.balance not updated in memory after DB update
2. Debt not created when needed - condition checked stale balance
3. Silent credit loss - missing else branch when remainingToConsume > 0

Solution: Use effectiveBalances Map as single source of truth for all
grant balance modifications across both passes (debt repayment and
consumption).

Also adds comprehensive test coverage:
- 8 debt creation edge case tests
- 12 consumption code path tests
- Total: 81 billing tests, 196 assertions

All tests pass. Reviewed by 4 CLI agents (Codebuff, Codex, Claude Code, Gemini).

Author: brandonkachen