Carry Drip unsubscribes onto DonorPerfect donor records

**Objective:** Find Drip subscribers who have unsubscribed and set the DonorPerfect flag `{flag}` on the matching donors, so the donor database records the email opt-out. Check at most `{max-subscribers}` subscribers (default 100).

**Step 1: List unsubscribed people in Drip**

Call `drip-get-subscriber` with `status` set to unsubscribed and `max-records` set to the limit.

**Step 2: Match them in DonorPerfect**

For each subscriber, call `donorperfect-get-donor` with `email` set to their email, falling back to `donorperfect-get-address` with the same email. For each donor found, call `donorperfect-get-flag` with `donor-id` set to the donor's id and drop donors who already carry `{flag}`.

**Step 3: Confirm**

Call `donorperfect-get-code` with `code` set to `{flag}` to check the flag code exists, and warn the user if it does not. Then show the donors to flag (donor-id, name, email) and the counts of subscribers not found in DonorPerfect and donors already flagged. Ask the user to confirm. Do not continue without an explicit yes.

**Step 4: Flag and confirm**

For each confirmed donor, call `donorperfect-set-flag` with `donor-id` and `flag` set to `{flag}`. Then call `donorperfect-get-flag` for each donor again to confirm, and report flagged, failed (with reasons) and skipped counts.

**Rules & Guidelines:**

* `drip-get-subscriber` is on the drip server. `donorperfect-get-donor`, `donorperfect-get-address`, `donorperfect-get-flag`, `donorperfect-get-code` and `donorperfect-set-flag` are on the donorperfect server.
* The only write this prompt may make is `donorperfect-set-flag`, on the donorperfect server, after confirmation in Step 3.
* Do not change anything in Drip: never call `drip-update-subscriber`, `drip-add-subscriber` or `drip-unsubscribe-subscriber`. Do not call `donorperfect-save-donor`.
* Match on email only. If one email matches several donors, list them and skip that email rather than flagging all of them.

**Validation:**

* If Drip has no unsubscribed people, say so and stop.
* If none of them is a DonorPerfect donor, say so; that is a valid result.