Support pgvector for RAG/agent use

Support a pgvector index of pages with:

• Configurable embedding model providers (including open source)
• Configurable chunking strategies
• Configurable debouncing of index updates
• Search API
• MCP availability

This would allow agents to search pages for standard RAG chat but also to potentially make page edits and do restructuring based on content (and to clean up/restructure old "classical" documents into a modern wiki design).