Adds step-level checkpoint storage so workflows can resume from the
last completed step after a crash or restart without replaying prior work.
- Migration: `workflow_checkpoints` table — workspace_id (FK + CASCADE),
workflow_id, step_name, step_index, completed_at, payload JSONB.
UNIQUE(workspace_id, workflow_id, step_name) + covering index on
(workspace_id, workflow_id, completed_at DESC).
- Handlers (platform/internal/handlers/checkpoints.go):
POST /workspaces/:id/checkpoints — upsert via ON CONFLICT DO UPDATE
GET /workspaces/:id/checkpoints/:wfid — list steps ordered step_index DESC
DELETE /workspaces/:id/checkpoints/:wfid — clear on clean shutdown (404 if none)
- Router: all three routes on the wsAuth group (WorkspaceAuth middleware);
workspace A's token cannot reach workspace B's checkpoints.
- Tests (11 cases, sqlmock + race-safe): upsert-insert, upsert-update,
payload forwarding, list-ordered, list-not-found, rows.Err() → 500,
delete-success, delete-not-found, callerMismatch 403 on all 3 endpoints.
Closes#788. Parent: #583-1.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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