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/base.html

@@ -139,10 +139,11 @@
   <header class="app-header">
     <div class="brand">Cassandra</div>
     <nav>
-      <a href="/"       class="{% if request.url.path == '/' %}active{% endif %}">Dashboard</a>
-      <a href="/news"   class="{% if request.url.path == '/news' %}active{% endif %}">News</a>
-      <a href="/log"    class="{% if request.url.path.startswith('/log') %}active{% endif %}">Log</a>
-      <a href="/upload" class="{% if request.url.path == '/upload' %}active{% endif %}">Import</a>
+      <a href="/"        class="{% if request.url.path == '/' %}active{% endif %}">Dashboard</a>
+      <a href="/news"    class="{% if request.url.path == '/news' %}active{% endif %}">News</a>
+      <a href="/log"     class="{% if request.url.path.startswith('/log') %}active{% endif %}">Log</a>
+      <a href="/upload"  class="{% if request.url.path == '/upload' %}active{% endif %}">Import</a>
+      <a href="/settings" class="{% if request.url.path == '/settings' %}active{% endif %}">Settings</a>
     </nav>
     <div class="header-right">
       <div id="tone-toggle" class="tone-toggle" data-tone="INTERMEDIATE"