feat: add ScriptBuilderSession model, migration, and schemas

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

backend/alembic/env.py

@@ -24,6 +24,7 @@ from app.models.ai_session import AISession  # noqa: F401
 from app.models.ai_session_step import AISessionStep  # noqa: F401
 from app.models.psa_post_log import PsaPostLog  # noqa: F401
 from app.models.psa_member_mapping import PsaMemberMapping  # noqa: F401
+from app.models.script_builder_session import ScriptBuilderSession  # noqa: F401
 from app.core.config import settings
 
 # this is the Alembic Config object