Drive user index engine from dedicated management stream with transactional writes

[qodo-code-review]

Description

• Avoid expensive log scan during startup to build user index management event index, which blocks custom indexes from working on large databases
• Eliminate extra $all subscription cost when custom indexes are not in use
• Enable atomic writes of index management events to both the individual index stream and a dedicated management stream
• Simplify event state management by extracting index identity from events instead of stream names

Deliverables

• Multi-stream writes: Extend SystemClient to support transactional writes to multiple streams with per-stream expected versions
• Management stream: Create $all-indexes stream to collect all user index management events, written transactionally with individual index streams
• Event store decorator: Implement UserIndexEventStore to intercept and duplicate index management events to the management stream
• Subscription refactor: Update UserIndexEngineSubscription to subscribe to management stream instead of building index from bootstrap
• Optional services: Make secondary indexing services optional with graceful error handling when disabled
• State simplification: Remove MultiEntityState abstraction and extract index names from events instead of stream names

[linear]

DB-1829