Restart a flow execution from step 1 with the original trigger inputs

Loads the IFlowExecution document for the supplied executionId, reads its inputs (the original trigger payload), and re-fires the flow as a NEW execution with a fresh executionContext.

Unlike /flow/resume/:executionId (which picks up from the last checkpoint), this endpoint starts over. Use when the failure was something fundamentally wrong about the run — bad data, corrupted intermediate state, deliberate “do it again” intent.

The SSE stream is prefixed with a FLOW_RESTARTED marker event so the client can render a visible “Restarted from step 1” divider in the chat between the failed run and the new run.