phase D milestones 1+2: referral system + paid-access gate

Lays the billing-prep spine before Paddle lands in D.3.

D.1 — referrals
- users.referral_code: unique 8-char URL-safe code (alphabet excludes the
  ambiguous 0/O/1/I/L). Generated lazily on first /settings hit so existing
  accounts pick one up without a backfill migration.
- users.referred_by_user_id + new referrals audit table (referrer,
  referred, created_at, converted_at, credited_at). converted_at /
  credited_at stay null until D.3 fills them via the Paddle webhook.
- POST /login accepts ?ref=<code>; the code rides on the signed
  pending-verify cookie so it survives the GET → POST → /verify hop.
- /settings page: email, tier badge, referral code chip + invite link
  with one-click copy, pending/converted/active-credits stats grid.
  Settings nav link added to the top bar.

Reward shape: when the referred user makes their first paid Paddle
subscription, both they and the referrer get 50% off for 3 months.
(D.3 wires the actual credit application via the Paddle webhook.)

D.2 — paid-access gate
- users.credit_until: timestamp until which a free-tier account has
  paid-tier access. Null = no credit. Populated by admin CLI now and the
  D.3 webhook later.
- app.services.access exposes paid_status(user) → PaidStatus dataclass
  (active / source / expires_at / days_remaining), is_paid_active() with
  admin-bearer-token bypass, and a require_paid FastAPI dependency that
  raises 402 Payment Required for free-tier callers.
- POST /api/analyze (portfolio AI commentary) gated behind require_paid.
- Settings page surfaces credit window when active ("free · credit · N
  day(s) remaining (expires YYYY-MM-DD)") and the upgrade hint when not.
- Admin CLI: python -m app.cli {grant-credit,revoke-credit,show-status}.
  grant-credit is idempotent — extends from max(now, current expiry) so
  re-running the command never erodes an existing grant.

Migrations 0013 (referrals) and 0014 (credit_until). Tests cover the
paid-status truth table, code generation + normalisation, CLI argument
parsing, and the pending-cookie ref roundtrip (29 new tests).

app/templates/login.html

@@ -18,6 +18,14 @@
     <div class="auth-card__brand">Cassandra</div>
     <div class="auth-card__hint">sign in with email</div>
 
+    {% if referrer_present %}
+    <div class="auth-info auth-info--invited">
+      <strong>You've been invited.</strong>
+      When you subscribe, you and your friend both get
+      <strong>50% off for 3 months</strong>. Sign up below to lock it in.
+    </div>
+    {% endif %}
+
     <p class="auth-card__lede">
       Enter your email and we'll send you a 6-digit code. No password.
       First-time visitors get an account; returning visitors get a sign-in.
@@ -27,6 +35,7 @@
 
     <form method="post" action="/login" autocomplete="on">
       <input type="hidden" name="next" value="{{ next_path or '/' }}">
+      {% if ref %}<input type="hidden" name="ref" value="{{ ref }}">{% endif %}
       <label>Email
         <input type="email" name="email" value="{{ email or '' }}" required autofocus>
       </label>