fix(billing): send org owner email on cloud trial start to backfill legacy billing email

[mekilis]

Summary

• Legacy cloud orgs created before billing email was synced have no billing email on the billing service, so customer creation fails during the trial mint and the org is left with a stamped trial claim but no subscription.
• StartTrial now attaches the org owner's email to the trial request, overwriting any client-supplied value. The billing service backfills its own blank record before minting.
• Best-effort by design: the billing service stays the single authority on whether an email is required and fails closed only when its own record is blank, so a lookup miss here is not fatal and cannot block orgs that already have an email.

Notes

• The billing service side (backfill before the trial claim, plus releasing a stuck claim on deterministic pre-mint failures) is handled in the billing service repo and deployed separately.
• StartTrialRequest gains a billing_email field for this.

Test plan

• go build ./... and go vet ./api/handlers/... pass (verified locally).
• Start a cloud trial on an org whose billing record has a blank billing email and confirm the mint succeeds and the trial pill/entitlements appear.
• Start a cloud trial on an org that already has a billing email and confirm it still succeeds (email not overwritten).

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Note

Medium Risk
Touches billing trial onboarding and forwards PII (owner email); behavior is narrow and aligned with existing ensureOrganisationInBilling / updateBillingEmailIfEmpty patterns, with billing service as final gate.

Overview
Cloud trial start now always includes the org owner’s email on the billing StartTrial request so legacy orgs with a blank billing email can complete customer creation during the trial mint instead of failing with a stamped claim but no subscription.

The handler sets requestData.BillingEmail from getOwnerEmail after decoding the body, which replaces any client-supplied billing_email. Lookup is best-effort (empty on miss does not block the request); the billing service remains authoritative on whether an email is required.

StartTrialRequest gains an optional billing_email JSON field and updated godoc. Swagger docs.go is regenerated only.

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[linear]

PDE-885