referrals: close D.3 — both parties get 45 days credit on conversion

The referral feature was half-built: codes captured, banner shown,
counts displayed — but no money flowed when a referred user paid.
The Settings page hard-coded "— (D.3)" for Active credits and the
marketing copy promised "50% off for 3 months" with nothing behind it.

Closing the loop:

- New `convert_referral(session, user)` in referral_service.py looks
  up the user's Referral row, stamps `converted_at` + `credited_at`,
  and extends `credit_until` by 45 days on BOTH the buyer and the
  referrer. Idempotent — replayed webhooks and renewals are no-ops.
  Stacks correctly when the user already has a credit window running
  (anchors at max(now, current_credit_until) like cli.grant_credit).

- Stripe webhook wires this into `_grant_paid`. A captured
  `first_paid_transition = user.tier != "paid"` gate avoids the DB
  lookup on every renewal event; convert_referral's own idempotency
  is the second line of defence.

- `_grant_paid` now takes `session` as its first positional arg so
  the conversion runs inside the same transaction as the tier flip
  and audit-row write. A mid-flight failure rolls everything back
  together — no partial state.

- Settings page replaces the "— (D.3)" placeholder with the live
  count of conversions still inside their 45-day credit window, plus
  a "+N days on your account" hint when the user has any credit of
  their own (referrer bonus, admin grant, or future refund-as-credit).

- Marketing copy on pricing.html + settings.html switches from "50%
  off for 3 months" to "45 days of paid access" — same economic value,
  honest about the actual mechanism (full free access rather than
  discounted billing).

Credit-amount rationale: 50% × 3 months ≈ 1.5 months of free
service ≈ 45 days. Pure-credit delivery is processor-agnostic, needs
no Stripe coupon plumbing, and stacks cleanly across referrals.

7 new tests in test_referral_conversion.py cover the happy path,
idempotency, no-referral no-op, credit stacking, deleted-referrer
survival, end-to-end webhook → credit landing, and the renewal-event
no-double-credit guarantee.

Also bundled: the Restore-button class fix from earlier
(portfolio.js — the cloud-restore "Restore" submit was unstyled and
picked up browser defaults; now uses .settings-btn like the rest of
the action-button family).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/templates/settings.html

@@ -121,7 +121,8 @@
       <summary class="settings-section__head">Invite a friend</summary>
       <p class="settings-section__lede">
         Share your invite link. When your friend subscribes, you and
-        they each get <strong>50% off for 3 months</strong>.
+        they each get <strong>45 days of paid access</strong> credited
+        to your account.
       </p>
 
       <div class="invite-block">
@@ -146,7 +147,12 @@
         </div>
         <div>
           <div class="invite-stats__label">Active credits</div>
-          <div class="invite-stats__value settings-row__hint">— (D.3)</div>
+          <div class="invite-stats__value">{{ active_credit_count }}</div>
+          {% if own_credit_days %}
+            <div class="settings-row__hint" style="margin-left:0;">
+              +{{ own_credit_days }} day{{ '' if own_credit_days == 1 else 's' }} on your account
+            </div>
+          {% endif %}
         </div>
       </div>
     </details>