Agents: CRUD, DB table, session binding, replace model picker

- agents table with name, avatar, description, model, persona, instructions, soft-delete
- AgentEndpoint (GET/POST/PUT/DELETE /api/agents)
- sessions + chains now reference agent_id
- ChatEndpoint resolves agent from session (no agent = no chat)
- system prompt built from agent persona + instructions
- frontend: agent editor with avatar upload, agent cards on dashboard
- session: agent dropdown replaces model picker, choice persisted

Program.cs

@@ -22,5 +22,6 @@ app.UseStaticFiles();
 app.MapChatEndpoint();
 app.MapHistoryEndpoint();
 app.MapSessionEndpoints();
+app.MapAgentEndpoints();
 
 app.Run("http://0.0.0.0:19000");