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>

migrations/versions/e7a15f49bc01_assessment_entered_at.py

@@ -0,0 +1,34 @@
+"""assessment entered at
+
+Revision ID: e7a15f49bc01
+Revises: 3b6f28fbdc4b
+Create Date: 2026-08-03 13:31:01.094907
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'e7a15f49bc01'
+down_revision: Union[str, None] = '3b6f28fbdc4b'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table('calls', schema=None) as batch_op:
+        batch_op.add_column(sa.Column('assessment_entered_at', sa.DateTime(), nullable=True))
+
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    with op.batch_alter_table('calls', schema=None) as batch_op:
+        batch_op.drop_column('assessment_entered_at')
+
+    # ### end Alembic commands ###