Remove unused listserv pipeline workflow

[DIodide]

Summary

Removes the unfinished listserv email ingestion pipeline runtime from the app:

• deletes the scheduled Pipeline Poll GitHub Actions workflow
• removes the FastAPI /pipeline router and implementation modules
• removes the pipeline-only admin web workspace
• drops pipeline-only env examples and backend dependencies
• removes unused listserv_configs / pipeline_logs schema exports while keeping the listserv_emails archive table used by scraper/import tooling

Motivation

The Pipeline Poll workflow was intended to periodically hit /pipeline/poll for an email/listserv ingestion system. That system was never fully implemented or configured, so the workflow was effectively dead code. On the old DIodide/TheForum fork, it was running on a schedule with missing secrets and repeatedly failing by calling an empty FASTAPI_URL (/pipeline/poll), creating noisy GitHub failure notifications.

Since future work should happen in TigerAppsOrg/TheForum, this PR removes the workflow and the dead runtime surface from the organization repo so the unfinished pipeline cannot be accidentally reactivated there. The standalone listserv scraper/archive work is left intact because it is separate from the broken scheduled FastAPI pipeline.

Operational note: I also manually disabled the active Pipeline Poll workflow on DIodide/TheForum to stop the existing notification loop.

Verification

• uv run python -c "from app.main import app; print([route.path for route in app.routes])"
• uv run ruff check app
• bun --filter @the-forum/database check-types
• bun --filter @the-forum/web check-types