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/routers/stripe_billing.py

@@ -232,6 +232,7 @@ async def _find_user(
 
 
 async def _grant_paid(
+    session: AsyncSession,
     user: User,
     *,
     customer_id: str | None,
@@ -239,6 +240,11 @@ async def _grant_paid(
     trial_end: int | None = None,
     status: str | None = None,
 ) -> None:
+    # Capture "first paid transition" before mutating — drives the
+    # referral-conversion call below. Skipping the convert lookup on
+    # every renewal event saves a DB roundtrip per webhook.
+    first_paid_transition = user.tier != "paid"
+
     user.tier = "paid"
     if customer_id and user.stripe_customer_id != customer_id:
         user.stripe_customer_id = customer_id
@@ -253,6 +259,13 @@ async def _grant_paid(
     elif status == "active":
         user.stripe_trial_end_at = None
 
+    # Apply referral credit on the FIRST paid transition only.
+    # convert_referral is itself idempotent (no-op on missing or
+    # already-converted rows), so this guard is purely a perf hint.
+    if first_paid_transition:
+        from app.services.referral_service import convert_referral
+        await convert_referral(session, user)
+
 
 async def _revoke_paid(user: User) -> None:
     user.tier = "free"
@@ -277,6 +290,7 @@ async def _handle_checkout_completed(
     # after will carry it. We grant paid here without trial info and
     # let the subscription event fill in trial_end_at moments later.
     await _grant_paid(
+        session,
         user,
         customer_id=obj.get("customer"),
         subscription_id=obj.get("subscription"),
@@ -301,6 +315,7 @@ async def _handle_subscription_event(
     # subscription.deleted (which fires after the final state lands).
     if status in ("trialing", "active"):
         await _grant_paid(
+            session,
             user,
             customer_id=obj.get("customer"),
             subscription_id=obj.get("id"),