fix(webapp): gate runTableV2 on native realtime instead of merging Electric shapes

Electric realtime shapes are bound to a single table, so a task_run_v2 run was invisible to realtime subscriptions. The previous approach merged two Electric shapes per tag/batch feed under a composite cursor, which doubled Electric long-poll connections for those feeds. Electric is being retired in favor of the native realtime backend, which is table-agnostic and already observes both run tables, so that merge is throwaway.

Drop the Electric dual-shape merge (revert realtimeClient to its single-table form, remove the merge module) and instead gate runTableV2 on the native backend: a run only routes to task_run_v2 when the deployment has native realtime enabled and the org's realtimeBackend flag is native. This keeps v2 runs realtime-observable without touching Electric, and the gate auto-satisfies once Electric is removed and native is the default. The idempotency pre-gate claim inherits the same gate.

Author: d-cs