stripe: per-cadence cooling-off + manage-subscription button

Bundles three related pieces that came out of the operator's first
end-to-end test of the paid flow:

1. Manage subscription button on /settings (paid users with a real
   Stripe sub — i.e. not credit-granted access). POSTs to the existing
   /api/stripe/portal endpoint; Stripe-hosted customer portal handles
   card updates, cancellation, monthly↔annual switch, invoice history.
   Replaces the stale "Paid features unlock with Paddle (D.3) or
   invite credits" hint for free users with a live link to /pricing.

2. Per-cadence cooling-off treatment:

   - **Annual £70**: 14-day free trial via
     subscription_data.trial_period_days=14. No money moves during
     the trial, so the CCR 2013 14-day refund question doesn't arise
     (nothing paid = nothing to refund). Card is still required at
     checkout so Stripe can charge on day 15.

   - **Monthly £7**: bills immediately. A 14-day trial there would
     give away ~50% of cycle one. Instead, /pricing now carries a
     required tick-box above the Subscribe buttons (subscribe stays
     disabled until checked) — by ticking, the user expressly
     consents to begin performance immediately and acknowledges that
     this extinguishes their statutory 14-day right under Reg 36
     CCR 2013. Consent collected on our own page (not via Stripe's
     account-wide consent_collection.terms_of_service) so each
     product can keep its own Terms URL as we add more.

3. T&C §6 clause 1 split into 1a (annual / trial substitute) +
   1b (monthly / Reg 36 waiver via on-page tick-box). Clause 2
   (post-cooling-off cancellation) unchanged.

Settings page shows "Free trial — N days remaining" while the
sub is in `trialing` status, falling back to "Paid subscription
active." once it transitions to active. Countdown is computed
server-side from User.stripe_trial_end_at (new column, migration
0020) populated by the subscription.created/updated webhook from
the Stripe trial_end timestamp; cleared on the trialing→active
transition and on revoke.

Drive-by: fixed a structlog kwarg-name collision on
`log.warning(..., event=event_type, ...)` in both polar_webhook.py
and stripe_billing.py — `event` is structlog's positional event
name and "got multiple values" crashed the user-not-found log
path. Renamed to `event_type=` everywhere it appeared. Caught by
the new trialing-stores-trial-end test.

Tests
- 4 new in test_stripe_billing.py covering monthly (no trial, no
  consent_collection), annual (trial, no consent), trialing stores
  trial_end, trialing→active clears trial_end.
- 1 existing test renamed + reworked for the consent split.
- Full suite: 224 passed, 5 skipped.

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

app/routers/pages.py

@@ -154,6 +154,21 @@ async def settings_page(
         .limit(1)
     )).scalar_one_or_none()
 
+    # Trial countdown — when the Stripe subscription is in its 14-day
+    # trial, show "N days remaining" on the tier row. Computed here
+    # rather than in the template because Jinja's date arithmetic is
+    # painful, and we already have to handle MariaDB's tz-naive
+    # round-trip via _aware-style normalisation.
+    trial_days_remaining: int | None = None
+    if user.stripe_trial_end_at is not None:
+        end = user.stripe_trial_end_at
+        if end.tzinfo is None:
+            end = end.replace(tzinfo=timezone.utc)
+        delta = end - datetime.now(timezone.utc)
+        if delta.total_seconds() > 0:
+            # Round up so the last hours of the trial still read "1 day".
+            trial_days_remaining = max(1, -(-int(delta.total_seconds()) // 86400))
+
     return templates.TemplateResponse(
         request, "settings.html",
         {
@@ -163,5 +178,6 @@ async def settings_page(
             "converted_count": int(converted_count),
             "paid": paid_status(user),
             "last_email_send": last_email_send,
+            "trial_days_remaining": trial_days_remaining,
         },
     )