Fix populate_db notebook for production seeding

[androemeda]

Reworks backend/populate_db.ipynb to seed the deployed Qdrant KB end-to-end.

Changes:

• Point uploads at the deployed backend (vcell-ai-dev.cam.uchc.edu/api) instead of localhost
• Load admin JWT from .env and attach as Bearer header on every request
• Delete-before-upload per filename for idempotent re-runs (backend has no dedup)
• Pass source_url on each upload so origin is preserved in the Qdrant payload
• Clean up noisy prints and stray Colab-isms

Seeded the dev KB with 153 files across the three configured source directories.