feat: add Google Workspace SSO configuration per school

- Implemented Google SSO management in the school settings, allowing schools to configure their own OAuth2 credentials.
- Added fields for Client ID and Client Secret in the edit school modal and school detail page.
- Introduced functionality to save and clear Google SSO settings via API.
- Updated UI to display current SSO status and instructions for setting up Google OAuth2.
- Created a new database migration to add `google_client_id` and `google_client_secret` columns to the schools table.

backend/migrations/versions/0003_school_google_sso.py

@@ -0,0 +1,29 @@
+"""schools: voeg Google Workspace SSO credentials toe per school
+
+Revision ID: 0003
+Revises: 0002
+Create Date: 2026-03-03
+
+Elke school heeft zijn eigen Google Workspace omgeving en dus
+zijn eigen OAuth2 client_id en client_secret. Deze worden per school
+opgeslagen en nooit blootgesteld via de API (enkel of ze ingesteld zijn).
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision      = '0003'
+down_revision = '0002'
+branch_labels = None
+depends_on    = None
+
+
+def upgrade():
+    op.add_column('schools',
+        sa.Column('google_client_id', sa.String(255), nullable=True))
+    op.add_column('schools',
+        sa.Column('google_client_secret', sa.String(255), nullable=True))
+
+
+def downgrade():
+    op.drop_column('schools', 'google_client_secret')
+    op.drop_column('schools', 'google_client_id')