FinancialDbContext uses global LiteDB BsonMapper with concurrency race risk

[simplerjiang]

• Source: .automation/tasks.json and .automation/sprint.md V041-DEBT-1.
• Impact: Concurrent worker tests or runtime initialization may race on global mapper registration.
• Repro:
  1. Run FinancialWorker tests sequentially: currently passes.
  2. Run parallel/concurrent initialization scenarios.
• Expected: Mapper registration is isolated per context or otherwise concurrency-safe.
• Actual: Debt remains because BsonMapper.Global is shared.
• Claim: Assign financial-worker owner.
• Repair: Inject private BsonMapper or lock initialization deliberately.
• Verification: Add concurrency test; run FinancialWorker.Tests.