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>

pyproject.toml

@@ -10,13 +10,15 @@ license = {text = "AGPL-3.0-or-later"}
 authors = [{name = "Benjamin Mako Hill", email = "mako@atdot.cc"}]
 requires-python = ">=3.11"
 # Known-good versions: flask 3.1.1, SQLAlchemy 2.0.40, alembic 1.13.2,
-# pylti1p3next 2.0.2 (development uses Debian system packages for all
-# but pylti1p3next; these floors record what the code was written against).
+# numpy 2.2.4, pylti1p3next 2.0.2 (development uses Debian system
+# packages for all but pylti1p3next; these floors record what the code
+# was written against).
 dependencies = [
     "flask>=3.1",
     "pylti1p3next>=2.0",
     "SQLAlchemy>=2.0",
     "alembic>=1.13",
+    "numpy>=1.26",
 ]
 
 [project.optional-dependencies]