Integrate platform simulators (Reddit / Twitter / Engram) as selectable environments

[PRATHAMESH75]

Context

Platform simulation scripts exist but are CLI-only: backend/scripts/run_reddit_simulation.py, run_twitter_simulation.py, run_engram_simulation.py, run_parallel_simulation.py.

Proposed work

• Promote these from scripts to UI-selectable simulation environments wired into the campaign flow.

Acceptance criteria

• User can choose a platform environment when starting a campaign
• Results flow into the standard dashboard/report