Phase 1: scaffold, data models, selection logic, tests

Flask application skeleton with SQLAlchemy models for courses (including
per-course settings), students, enrollments, opt-outs, and calls; the
weighted and cycle selection logic ported from the manual coldcall
scripts; alembic migrations; and a pytest suite covering selection
behavior and the model query helpers.

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

tests/conftest.py

@@ -0,0 +1,15 @@
+import pytest
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+
+from coldcall_lti.db import Base
+
+
+@pytest.fixture
+def db_session():
+    engine = create_engine("sqlite://")
+    Base.metadata.create_all(engine)
+    session = sessionmaker(bind=engine)()
+    yield session
+    session.close()
+    engine.dispose()