📝 docs(converter): fix critical design flaws in v1.2

[jorben]

Update ConverterWorker design document to v1.2 with major fixes:

• Remove FOR UPDATE SKIP LOCKED (SQLite incompatible) → use Prisma optimistic locking
• Fix Model.ts import mismatch → use default import
• Fix retry count timing → increment before each attempt
• Add task cancellation check → filter CANCELLED tasks in claimPage
• Add transaction conflict retry → handle Prisma P2034 errors
• Improve token extraction → support multiple providers (OpenAI/Claude/Gemini/Ollama)
• Reduce streaming update frequency → 2s throttle to minimize DB writes
• Remove unused successProgress concept
• Add content validation logic
• Optimize error classification with typed exceptions