Vector store maintenance

Keep the retrieval index synchronized with the source of truth.

Stale vectors produce stale answers.

#### Scheduled refresh

Scheduled job
   ↓
Detect changes
   ↓
Re-index

#### Event-driven refresh

Document change
   ↓
Event
   ↓
Processing
   ↓
New embedding
   ↓
Index update

#### Incremental update

Only changed documents/chunks are reprocessed.

Rebuilding the entire vector index for every document change may be unnecessarily expensive.

Prefer incremental synchronization when appropriate.

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