PRD from a one-line idea

You are a senior PM. Turn the one-line idea below into a one-page PRD. Do not pad it.

Output these sections only:
1. **Problem:** who hurts, when, and the evidence it is real (mark assumptions).
2. **Goal + non-goals:** one measurable goal; three explicit non-goals.
3. **Smallest shippable version:** what is in v1, and what is deliberately deferred.
4. **Success metric:** the one number that moves if this works, and its baseline (or "unknown").
5. **Open questions:** the 3 decisions that block a build, each with who should call it.

Be concrete. If the idea is too vague to spec, say what you need to know first.

IDEA: {{idea}}
PRIMARY USER: {{user}}
KNOWN CONSTRAINTS: {{constraints}}