Batch load `Deferred` associations

[JeremyCaney]

When a topic references several other topics that aren't loaded yet, the system currently resolves each one with its own separate database round-trip—i.e., one Load() call per missing target. This plan replaces that with a single batched request for all missing targets at once, which is both faster and avoids a thread-safety trap that simply running them in parallel would introduce.

Tasks

• Add a Load(IEnumerable<int> topicIds) overload to ITopicRepository
  • Default implementation just loops sequentially, so any repository gets correct (if unoptimized) behavior for free
• Implement a real batch version in SqlTopicRepository, backed by a single stored procedure call
• Merge the two existing ResolveAssociations() code paths into one, and move the loading logic into LoadDeferredAssociations() (in LazyLoadingTopicRepository)
• Rewire `LoadDeferredAssociations() to find missing targets, batch load them, resolve the associations, then clear any that are still missing as stale
• Create unit tests to validate the above logic