Backfill existing Flodesk subscribers into any automation

Build me an app for backfilling my Flodesk automations. Flodesk only offers the "include existing subscribers" option at the exact moment you publish or republish a workflow, and re-adding someone to a segment they are already in does not trigger anything, so my long-time subscribers silently never receive my welcome or nurture sequences. This app is where I go to fix that: choose an automation, choose a segment, and enroll the people who missed it.

Layout: a left rail lists my Flodesk automations using List Workflows, showing each workflow's name and status, with a filter so I can look at published ones only. Selecting a workflow opens the main pane for that workflow. Keep the selected workflow in the URL so I can link straight back to one.

The main pane has a segment picker populated from List All Segments. Once I pick a segment, load its subscribers with List All Subscribers filtered by that segment, and split them into two tabbed or side by side lists: "Already enrolled by this app" and "Never enrolled". Each row shows email, first and last name, subscriber status, and for already-enrolled rows the date this app enrolled them. Use Retrieve Subscriber when I open a single row for detail or want to refresh one person.

Enrollment tracking: Flodesk exposes no read operation for workflow membership, so the split above depends on a custom field the app maintains. On first run, check List All Custom Fields for the app's tracking field and create it with Create Custom Field if it is missing. After each successful enrollment, stamp the subscriber with Create or Update Subscriber, recording which workflows this app has put them through and when. That stamp is the source of truth for the already-enrolled list, so treat it carefully and never overwrite a subscriber's other custom field values when writing it.

Enrolling: I can tick individual rows or select the entire segment at once. Hitting Enroll first shows a dry-run preview, telling me how many people will be enrolled, how many are being skipped and why (already enrolled, unsubscribed, missing email), and listing the actual people. Confirming starts a throttled batch that calls Add Subscriber to Workflow per subscriber, with a live progress list showing each person moving from pending to enrolled or failed, a running count, and a way to stop mid-run. Failures should be listed with their reason and be retryable without re-running the whole batch.

Respect Flodesk's rate limit of 100 requests per minute so a large backfill does not die halfway through. Pace the batch off the X-Fd-RateLimit-Remaining header returned on every response, back off when it gets low, retry 429 responses rather than failing the run, and make runs resumable: persist progress as you go so an interrupted run picks up where it left off and never enrolls the same person twice. Pagination uses page and per_page with a maximum of 100 per page, and the response envelope returns meta.total_pages, so page through fully rather than showing only the first page.

Exclude unsubscribed people by default. Filter them out of the enrollable list, never include them when I select a whole segment, and give me a toggle to see them greyed out and unselectable if I want to confirm who was skipped.

Undo: from the same screen I can select people and pull them back out with Remove Subscriber from Workflow, which should also clear that workflow from their tracking stamp via Create or Update Subscriber so the lists stay honest.

History: keep a per-workflow history of every backfill run in the app's own storage. Each entry records the segment, who started it, when it ran, how many were enrolled, skipped and failed, and the full list of subscribers enrolled with timestamps. Show that history under the selected workflow so I can answer "who got put through this and when" months later, and let me open any past run to see its full roster.