feat(autograder): add live-API, pandas-transform, and architecture-overview checks

[lassebenni]

Summary

Three new automated checks, each corresponding to a minimum requirement that was previously unverified:

• HTTP library import (requests / httpx / aiohttp) — critical: a CSV-only pipeline with no API call would have passed before
• pandas real transform operations (.groupby, .agg, pd.to_datetime, .dt., .assign, .merge, .fillna, .dropna) — critical: pd.DataFrame(records) straight to storage would have passed before
• Architecture overview in README (mermaid block, ASCII arrows → ↓, or ## Architecture / ## Data flow heading): no check existed for this documentation requirement

Also includes minor cleanup: type annotations, from __future__ import annotations, consistent helper style, clearer check labels.

Test plan

• Open a student repo that uses requests + real pandas transforms + has a README architecture section — confirm all new checks pass
• Open a stub repo with no API import — confirm HTTP library check fails as critical
• Open a repo with only pd.DataFrame(records) — confirm transform check fails as critical