Env var hygiene: PORTAL_BACKEND_URL missing from .env.example; DYNAMODB_STUDENT_REPORTS_TABLE_NAME unused

[deepansh96]

Two environment-variable inconsistencies found while documenting the codebase:

1. PORTAL_BACKEND_URL is required at import time but missing from .env.example

app/auth/__init__.py reads it with os.environ["PORTAL_BACKEND_URL"] at module import, so a missing value crashes the whole app at startup (KeyError), not with a 500 on first use. On Lambda this surfaces as API Gateway 502s on cold start.

It currently works locally only because of import order in app/main.py: internal.db (which calls load_dotenv("../.env.local")) is imported before the router that pulls in auth. Reordering imports would break startup.

Fix: add PORTAL_BACKEND_URL= to .env.example. Optionally, read the var lazily (inside the verify function) so startup doesn't depend on import order.

2. DYNAMODB_STUDENT_REPORTS_TABLE_NAME is defined but never used

It's listed in .env.example, but app/db/reports_db.py hard-codes both table names (student_quiz_reports, student_quiz_reports_v2).

Fix: either wire the wrapper to read table names from env (which would also unblock a real staging table), or drop the var from .env.example to stop suggesting it does something.

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