fix: improve import, evidence indexing and material-scoped grading

Preserve document revisions and evidence, harden indexing and export behavior, add explicit material selection and direct-folder import, validate model JSON schemas with safe error messages, and retain student selection while loading scoring data.

backend/migrations/versions/0009_navigation_index.py

@@ -0,0 +1,21 @@
+"""New navigation policy requires rebuilding existing indexes explicitly.
+
+Revision ID: 0009
+Revises: 0008
+"""
+from alembic import op
+
+revision = "0009"
+down_revision = "0008"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    with op.batch_alter_table("knowledge_documents") as batch:
+        batch.alter_column("index_version", server_default="5")
+
+
+def downgrade():
+    with op.batch_alter_table("knowledge_documents") as batch:
+        batch.alter_column("index_version", server_default="4")