feat(indexer): scaffold entity-level SDLC indexing (ADR 014)

What does this MR do and why?

Scaffolding for entity-level SDLC indexing (#746 (closed), ADR 014). Adds message types, config, and no-op handler/dispatcher. Follow-up MRs wire in pipelines and partition logic on top of this.

• EntityIndexingRequest, IndexingScope, PartitionAssignment types in topic.rs
• EntityHandlerConfig / EntityDispatcherConfig with schema generation
• No-op EntityIndexingHandler (subscribes, deserializes, logs) and EntityDispatcher (runs on cron, publishes nothing)
• Registered alongside existing handlers; no behavior change

Related Issues

Relates to #746 (closed)

Testing

Unit tests for topic serialization roundtrips and handler deserialization. Full suite passes.

Performance Analysis

• This merge request does not introduce any performance regression. If a performance regression is expected, explain why.