Setup banner, assessment timestamps, and installation docs

Instructor home shows a setup banner while a course has no class
schedule, since that is the one gap that degrades the student
experience (free-date opt-outs, no withdrawal locking); it points at
the prefilled schedule form and settings and disappears once any
class day exists.

Calls gain assessment_entered_at, stamped when an outcome is recorded
or actually changed (bulk saves that change nothing don't restamp),
null for pending calls and legacy imports, and included in the calls
export — distinguishing assessed-in-class from assessed-later.

The README setup section now gives both install routes: pure
pip/venv, and Debian system packages with the apt line. Fixing this
uncovered that numpy was missing from the project dependencies (the
grading engine needs it; the system-site-packages venv had masked
the omission).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -68,17 +68,30 @@ roster.
 
 ## Setup
 
-Development uses a virtualenv that shares the system's Debian-packaged
-libraries (Flask, SQLAlchemy, alembic, pytest) and adds the one
-PyPI-only dependency, the maintained `pylti1p3next` fork of PyLTI1p3:
+The app needs Python ≥ 3.11 with Flask, SQLAlchemy, alembic, and
+numpy, plus one library that only exists on PyPI: `pylti1p3next`, the
+maintained fork of PyLTI1p3. There are two equivalent ways to install,
+depending on whether you prefer distribution packages or pip.
+
+**Pure pip/venv** (any distribution; everything from PyPI):
 
 ```
+python3 -m venv .venv
+.venv/bin/pip install -e '.[test]'
+```
+
+**Debian/Ubuntu system packages** (only `pylti1p3next` comes from
+PyPI; the venv shares the system's packages):
+
+```
+sudo apt install python3-venv python3-flask python3-sqlalchemy \
+    python3-alembic python3-numpy python3-pytest
 python3 -m venv --system-site-packages .venv
 .venv/bin/pip install pylti1p3next
 .venv/bin/pip install -e .
 ```
 
-Create the database and run the tests:
+Either way, then create the database and run the tests:
 
 ```
 .venv/bin/python -m alembic upgrade head