Migrate assessments to class-based model

- Dropped the existing assessments table to remove user_id association.
- Created a new assessments table linked to classes (class_id) allowing multiple teachers to share assessments.
- Added necessary indexes for performance optimization.
- Downgrade functionality to revert back to user-based assessments if needed.

backend/migrations/versions/0004_class_based_assessments.py

@@ -0,0 +1,60 @@
+"""assessments: herstructureer naar klasgebonden model
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-03-05
+
+Wijziging: assessments zijn niet langer gekoppeld aan een individuele
+leerkracht (user_id) maar aan een klas (class_id). Meerdere leerkrachten
+van dezelfde klas delen één set beoordelingen.
+
+OPGELET: dit dropt de bestaande assessments tabel — testdata gaat verloren.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision      = '0004'
+down_revision = '0003'
+branch_labels = None
+depends_on    = None
+
+
+def upgrade():
+    # Drop oude tabel volledig (testomgeving — geen productiedata)
+    op.execute("DROP TABLE IF EXISTS assessments CASCADE")
+
+    # Nieuwe tabel: klasgebonden, geen user_id
+    op.execute("""
+        CREATE TABLE assessments (
+            id             SERIAL PRIMARY KEY,
+            class_id       INTEGER NOT NULL REFERENCES classes(id)      ON DELETE CASCADE,
+            school_year_id INTEGER NOT NULL REFERENCES school_years(id) ON DELETE CASCADE,
+            vak_id         VARCHAR(50) NOT NULL,
+            goal_id        VARCHAR(50) NOT NULL,
+            status         VARCHAR(10) NOT NULL,
+            opmerking      VARCHAR(500),
+            updated_at     TIMESTAMP DEFAULT NOW(),
+            UNIQUE(class_id, school_year_id, vak_id, goal_id)
+        )
+    """)
+    op.execute("CREATE INDEX IF NOT EXISTS ix_assessments_class_year ON assessments(class_id, school_year_id)")
+    op.execute("CREATE INDEX IF NOT EXISTS ix_assessments_vak ON assessments(vak_id)")
+
+
+def downgrade():
+    op.execute("DROP TABLE IF EXISTS assessments CASCADE")
+    # Zet terug naar user-gebaseerde tabel (zonder data)
+    op.execute("""
+        CREATE TABLE assessments (
+            id             SERIAL PRIMARY KEY,
+            user_id        INTEGER NOT NULL REFERENCES users(id)        ON DELETE CASCADE,
+            school_id      INTEGER NOT NULL REFERENCES schools(id)      ON DELETE CASCADE,
+            school_year_id INTEGER NOT NULL REFERENCES school_years(id) ON DELETE CASCADE,
+            vak_id         VARCHAR(50) NOT NULL,
+            goal_id        VARCHAR(50) NOT NULL,
+            status         VARCHAR(10) NOT NULL,
+            opmerking      VARCHAR(500),
+            updated_at     TIMESTAMP DEFAULT NOW(),
+            UNIQUE(user_id, school_year_id, vak_id, goal_id)
+        )
+    """)