feat(knowledge): User ownership model for chat-derived knowledge #688

Summary

Implement user ownership and sharing model for knowledge base facts derived from chat conversations.

Problem

Currently, KB facts from chats are only tagged with session_id but have no user ownership or sharing model. When a conversation is shared, there's no mechanism to share the associated knowledge. Users cannot filter "my knowledge" vs "shared with me".

Proposed Solution

New Metadata Fields for Facts

metadata = {
    # Existing
    "source_session_id": "uuid",
    "category": "chat_knowledge",      # New category
    
    # NEW: Ownership
    "owner_id": "user_id",
    "visibility": "private",          # private | shared | public
    
    # NEW: Sharing
    "shared_with": ["user_id1", "user_id2"],
    "shared_via_session": "session_id",
    
    # NEW: Source tracking
    "source_type": "chat",            # chat | manual | import | system
    "created_by": "user_id",
}

New Redis Indexes

user:kb:facts:{user_id}           -> Set of fact IDs owned by user
user:kb:shared:{user_id}          -> Set of fact IDs shared with user
kb:category:chat_knowledge        -> Set of all chat-derived facts

Access Control

• Owner always has access
• Public facts accessible to all
• Check explicit shares list
• Check session-based sharing

Acceptance Criteria

• Facts created from chat are automatically tagged with owner_id
• Default visibility is "private"
• Access control enforced on all KB queries
• Redis indexes created for efficient user-based queries

Related Issues

• Depends on: Knowledge Base: Orphaned data from deleted conversations not cleaned up #547 (Session-fact relationship tracking) ✅ Complete
• Parent: Chat Knowledge Ownership Epic

Technical Notes

• Backend: src/knowledge/facts.py - Add ownership fields
• Backend: autobot-user-autobot-user-backend/api/knowledge.py - Add access control to queries
• Affects: Knowledge search, fact retrieval, orphan cleanup