refactor(A1): remove Flamenco, simplify render pipeline to Celery-only

- Remove flamenco-manager and flamenco-worker from docker-compose.yml
- Delete flamenco_client.py, flamenco_tasks.py, docker_scaler.py
- Simplify render_dispatcher.py to Celery-only (removes ~300 lines)
- Remove Flamenco beat schedule from celery_app.py
- Clean admin.py: remove flamenco settings, endpoints, threejs validation
- Clean orders.py cancel-render: Celery revoke only
- Clean worker.py: remove flamenco_job_id from activity response
- Migration 032: cancel lingering flamenco jobs, remove flamenco settings
- PLAN.md: mark all decisions confirmed, status IN UMSETZUNG

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/alembic/versions/032_remove_flamenco.py

@@ -0,0 +1,49 @@
+"""Remove Flamenco: cancel lingering flamenco jobs, clean up settings.
+
+Revision ID: 032
+Revises: 031
+Create Date: 2026-03-06
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = '032'
+down_revision = '031'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Cancel any order lines that were dispatched to Flamenco and never completed
+    op.execute("""
+        UPDATE order_lines
+        SET render_status = 'cancelled',
+            render_completed_at = NOW(),
+            render_log = render_log || '{"cancelled_reason": "flamenco_removed_in_v2"}'::jsonb
+        WHERE render_backend_used = 'flamenco'
+          AND render_status IN ('processing', 'pending')
+    """)
+
+    # Remove Flamenco-specific system settings
+    op.execute("""
+        DELETE FROM system_settings
+        WHERE key IN ('flamenco_manager_url', 'flamenco_worker_count')
+    """)
+
+    # Reset render_backend setting to 'celery' if it was 'flamenco' or 'auto'
+    op.execute("""
+        UPDATE system_settings
+        SET value = 'celery'
+        WHERE key = 'render_backend' AND value IN ('flamenco', 'auto')
+    """)
+
+
+def downgrade():
+    # Re-insert default Flamenco settings
+    op.execute("""
+        INSERT INTO system_settings (key, value, updated_at)
+        VALUES
+            ('flamenco_manager_url', 'http://flamenco-manager:8080', NOW()),
+            ('flamenco_worker_count', '1', NOW())
+        ON CONFLICT (key) DO NOTHING
+    """)