Add an opt-in named-entity extraction pass so the wiki knowledge graph
captures fine-grained entities (people, organizations, locations, ...)
and their relations, not just page-level link relations.
- new tables mate_wiki_entity / _mention / _relation (h2/mysql/kingbase)
- structured LLM extraction per chunk with entity resolution
(normalized-key dedup + embedding near-merge), mention/relation
persistence and page linking via chunk citations
- per-KB opt-in toggle (off by default); async dispatch after embedding
- read API: entity list, KB graph, entity ego-graph, manual extract
- UI: entity-layer toggle in the graph view + KB config toggle
- replace inline fully-qualified class names with imports in WikiProcessingService
Closes#336
