Date columns: make nullable, default NULL
MCPClient: use Django models

refs #7137
models: replaces needs db_column
TaskType: assign uuid to id, not pk
jobChainLink: print to stderr, don't print stderr
MCPServer: fix pk string references
MCPServer: fix nextmicroservicechainlink reference
MCPServer: tasktype => tasktype_id
MCPServer: fix StandardTaskConfig field names
MCPServer: fix dashboard import path