Proposal: scheduled (recurring) research reports for a search space

[mvanhorn]

Users today run research on demand. Khoj has Automations for this; AnythingLLM users are asking for it in #5349. Perplexity and NotebookLM don't have it.

I'd like to add a scheduled_research table + Celery-beat tick that runs the existing report.py agent on a cron schedule and emails the digest. Frontend exposes a "Schedule this report" button in the existing report-gen flow, and a list view at /dashboard/[search_space_id]/scheduled-research to manage/edit/disable schedules.

Scope (single PR): new table + Alembic migration, CRUD routes, 3 Celery tasks (tick / run / email), a modal + list page in the dashboard. Total ~500–700 LOC.

A few decisions I'd like your input on before I open the PR:

1. Celery-beat is the obvious fit given app/tasks/celery_tasks/ already exists — OK to add beat alongside the worker, or do you prefer a separate beat service per your deployment model?
2. Email delivery: reuse the SMTP path used by auth flows, or route through a new adapter in app/services/notifications/?
3. Cron safety: I'd enforce a minimum-granularity of daily (no sub-daily) in v1 to avoid runaway jobs while we tune. Agree?

Reference: I've shipped 3 frontend PRs on SurfSense (#1215, #1220, #1233) but this would be my first backend PR. I'll target dev and follow the migration + Alembic conventions already in alembic/versions/.