Admin Reseed Welcome

Wipe an existing handle's welcome thread(s) and re-seed with the current template.

Brian m_b4372290-follow-up (Leslie session 2026-04-25). The welcome template was retired mid-session — Leslie had already received the old chatty version. This endpoint backfills: deletes any existing welcome thread(s) that the well-known welcome address sent to this handle, then runs the seed function again so the user sees the new minimal welcome.

Why the matcher is conservative: only deletes threads where from_handle is the known welcome sender AND the subject matches a known welcome phrase (current OR previous). This intentionally doesn't cascade to "any thread between Brian and the user" because we don't want a future bug to wipe real conversation history.

Path: POST /v1/admin/handles/<handle>/reseed-welcome

Auth: admin API key.

Returns: {"ok": true, "handle": "leslie@tandem-cc.com", "deleted_threads": N, "reseeded": true}