CRITICAL: Remove cascade delete from Section→Quiz relationship

Root cause of lost PREP 2016 quiz: deleting a section (e.g. to recreate
with different page ranges) cascade-deleted ALL quizzes attached to it,
destroying the quiz AND all its questions permanently.

Fixes:
- Section.quizzes relationship: removed cascade="all, delete-orphan"
  Deleting a section no longer touches quizzes at all
- Quiz.section_id FK: changed to ON DELETE SET NULL, nullable=True
  If a section is deleted, quizzes keep working (section_id becomes null)
- DB migration: drops old FK constraint and recreates with SET NULL

Also answers the question about deleting a bank question in a quiz:
- quiz_question_links FK has ON DELETE CASCADE — deleting a question
  silently removes it from all quizzes
- This needs a warning UI (not yet implemented — no delete button in bank)

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -161,6 +161,14 @@ def setup_pgvector():
         # Fix email collation to avoid en_US.utf8 B-tree index corruption
         conn.execute(text('ALTER TABLE users ALTER COLUMN email TYPE varchar COLLATE "C"'))
 
+        # Fix: section deletion must not cascade-delete quizzes
+        conn.execute(text("ALTER TABLE quizzes ALTER COLUMN section_id DROP NOT NULL"))
+        try:
+            conn.execute(text("ALTER TABLE quizzes DROP CONSTRAINT IF EXISTS quizzes_section_id_fkey"))
+            conn.execute(text("ALTER TABLE quizzes ADD CONSTRAINT quizzes_section_id_fkey FOREIGN KEY (section_id) REFERENCES sections(id) ON DELETE SET NULL"))
+        except Exception:
+            pass  # constraint may already be correct
+
         # Soft delete + publish control for quizzes
         conn.execute(text("ALTER TABLE quizzes ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMP"))
         conn.execute(text("ALTER TABLE quizzes ADD COLUMN IF NOT EXISTS is_published INTEGER DEFAULT 1"))

backend/app/models/quiz.py

@@ -12,7 +12,7 @@ class Quiz(Base):
     __tablename__ = "quizzes"
 
     id = Column(Integer, primary_key=True, index=True)
-    section_id = Column(Integer, ForeignKey("sections.id"), nullable=False)
+    section_id = Column(Integer, ForeignKey("sections.id", ondelete="SET NULL"), nullable=True)
     user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
     category_id = Column(Integer, ForeignKey("quiz_categories.id", ondelete="SET NULL"), nullable=True)
     title = Column(String, nullable=False)

backend/app/models/section.py

@@ -14,4 +14,5 @@ class Section(Base):
     end_page = Column(Integer, nullable=False)
 
     document = relationship("PDFDocument", back_populates="sections")
-    quizzes = relationship("Quiz", back_populates="section", cascade="all, delete-orphan")
+    # No cascade delete — deleting a section must NOT destroy quizzes/questions
+    quizzes = relationship("Quiz", back_populates="section")