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).

tests/test_cli.py

@@ -0,0 +1,49 @@
+"""Unit tests for app.cli.
+
+Sub-command parsing only — the DB-touching paths (`grant_credit`,
+`revoke_credit`, `show_status`) are exercised manually inside the dev
+container. The parser-level tests are enough to catch the common
+shapes: bad args, missing args, unknown sub-command."""
+from __future__ import annotations
+
+import pytest
+
+from app.cli import build_parser
+
+
+def test_grant_credit_parses():
+    args = build_parser().parse_args(["grant-credit", "user@example.com", "3"])
+    assert args.cmd == "grant-credit"
+    assert args.email == "user@example.com"
+    assert args.months == 3.0
+
+
+def test_grant_credit_accepts_fractional_months():
+    args = build_parser().parse_args(["grant-credit", "user@x.com", "0.5"])
+    assert args.months == 0.5
+
+
+def test_revoke_credit_parses():
+    args = build_parser().parse_args(["revoke-credit", "user@example.com"])
+    assert args.cmd == "revoke-credit"
+    assert args.email == "user@example.com"
+
+
+def test_show_status_parses():
+    args = build_parser().parse_args(["show-status", "user@example.com"])
+    assert args.cmd == "show-status"
+
+
+def test_grant_credit_requires_months():
+    with pytest.raises(SystemExit):
+        build_parser().parse_args(["grant-credit", "user@example.com"])
+
+
+def test_unknown_command_rejected():
+    with pytest.raises(SystemExit):
+        build_parser().parse_args(["bogus-cmd"])
+
+
+def test_no_command_rejected():
+    with pytest.raises(SystemExit):
+        build_parser().parse_args([])