First Alembic revision for the DeerFlow application schema. Adds a
nullable users.default_workspace_id column with an ON DELETE SET NULL
FK to workspaces(id), so newly-registered users can be routed back to
their default workspace on next login without consulting memberships.
Uses op.batch_alter_table for SQLite ALTER compatibility (sqlite is
still a supported dev fallback).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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