feat(training): separate opt-in contribution receipts with withdrawable tombstones

A journal save now records no training contribution. A contribution is a second
consent: the privacy page states consent version training-consent-v1 and the
receipt (contribution id, included fields, never-included fields) before the
confirm control is enabled, the receipt is queued on this device only and copies
no photo bytes, note text, or symptom answers, and withdrawal keeps a tombstone
naming every derived artifact by contribution id. Delete-all clears the receipts
with the ledger.

Closes #24

docs/PRODUCT-DECISIONS.md

@@ -34,6 +34,14 @@ The application’s deterministic urgent-symptom guidance runs before chat and r
 
 Logging must never depend on AI availability. When the vision worker is offline, disabled, or still unknown, the photo-first flow offers no AI action and no analysis-consent prompt, and manual Bristol entry remains one action away from every photo step. A chosen photo and any entered context are retained as an in-session draft with a visible summary and an explicit start over; drafts are deliberately not persisted to storage, so a photo awaiting consent is never written to disk. The service worker never caches analysis requests or image bodies.
 
+### Training contribution consent
+
+Contributing a confirmed photo to training is a second consent, not a side effect of logging. A journal save is never a training contribution; a contribution exists only after the person reads the current consent version and its receipt (contribution id, what is included, what is never included) and accepts it explicitly. The receipt carries the consent version it was accepted under, the confirmed Bristol form and broad color, and no note text, symptom answers, urgency/discomfort ratings, identity, or photo bytes.
+
+Withdrawal is first-class: withdrawal keeps a tombstone naming every derived artifact by contribution id (raw image and confirmed label), so a deletion request can be addressed against the corpus rather than trusted to a promise. Delete-all clears the ledger, saved photos, and contribution receipts together.
+
+This build acknowledges that records the intent and does not transmit a contribution until the corpus pipeline and privacy copy are reviewed in #22 and #25; the section says so in the app rather than implying an upload that does not exist.
+
 ### Release authority
 
 Hermes/Timmy is the release authority for routine engineering, test, build, deployment, and release-candidate preparation. Automated evidence must remain reproducible and must not overstate model accuracy or safety.