Per-embedder attribution funnel: story → signup → paid.

For a single partner embedder, reads the full pull-through ladder inside the last days window:

1. stories_served — plays carrying this embedder's source_embedder_slug (the agent surface actually served a story that got listened to)
2. signups — listeners whose referred_by_embedder_slug is this embedder (a served story converted to a private-feed subscription)
3. paid — of those signups, how many are on a paid tier (plus / pro) now

The flywheel snapshot aggregates these across ALL embedders; this endpoint breaks them out per-partner so the operator can see which surfaces convert (and which just drive vanity plays). Conversion rates are stage-to-stage: signup_rate = signups / stories, paid_rate = paid / signups.

404 if the slug is not a registered embedder.