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_access.py

@@ -0,0 +1,133 @@
+"""Unit tests for app.services.access — the paid-tier gate.
+
+No DB; we hand-construct ``User`` rows and ``CurrentUser`` principals
+directly. The point is to nail down the truth table:
+
+    tier         | credit_until      | active | source
+    -------------|-------------------|--------|--------
+    free         | None              | False  | None
+    free         | past              | False  | None
+    free         | future            | True   | credit
+    paid         | None              | True   | tier
+    paid         | future            | True   | tier      (tier wins)
+    enterprise   | None              | True   | tier
+    admin bearer | n/a               | True   | (bypass)
+"""
+from __future__ import annotations
+
+from datetime import datetime, timedelta, timezone
+from types import SimpleNamespace
+
+import pytest
+
+from app.auth import CurrentUser
+from app.services.access import is_paid_active, paid_status
+
+
+def _utcnow() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+def _make_user(*, tier: str = "free", credit_until: datetime | None = None):
+    """Build something User-shaped without touching SQLAlchemy."""
+    return SimpleNamespace(tier=tier, credit_until=credit_until)
+
+
+# ---------------------------------------------------------------------------
+# paid_status — the truth table
+# ---------------------------------------------------------------------------
+
+
+def test_paid_status_free_no_credit():
+    st = paid_status(_make_user(tier="free"))
+    assert st.active is False
+    assert st.source is None
+    assert st.expires_at is None
+    assert st.days_remaining is None
+
+
+def test_paid_status_free_expired_credit():
+    st = paid_status(_make_user(tier="free", credit_until=_utcnow() - timedelta(days=1)))
+    assert st.active is False
+    assert st.source is None
+
+
+def test_paid_status_free_future_credit():
+    expiry = _utcnow() + timedelta(days=45)
+    st = paid_status(_make_user(tier="free", credit_until=expiry))
+    assert st.active is True
+    assert st.source == "credit"
+    assert st.expires_at == expiry
+    # Allow ±1 day slack for clock drift; integer-days floors.
+    assert 44 <= st.days_remaining <= 45
+
+
+def test_paid_status_paid_tier_no_credit():
+    st = paid_status(_make_user(tier="paid"))
+    assert st.active is True
+    assert st.source == "tier"
+    assert st.expires_at is None
+
+
+def test_paid_status_paid_tier_wins_over_credit():
+    """A paid subscription dominates — we surface 'tier' even if a
+    credit row also exists. Avoids confusing the user with 'X days
+    remaining' when they're actually on a rolling subscription."""
+    st = paid_status(_make_user(tier="paid", credit_until=_utcnow() + timedelta(days=10)))
+    assert st.source == "tier"
+    assert st.days_remaining is None
+
+
+def test_paid_status_enterprise_tier():
+    st = paid_status(_make_user(tier="enterprise"))
+    assert st.active is True
+    assert st.source == "tier"
+
+
+def test_paid_status_none_user():
+    """No DB row → no paid status. Admin bearer-token hits this path."""
+    st = paid_status(None)
+    assert st.active is False
+    assert st.source is None
+
+
+def test_paid_status_handles_naive_datetime():
+    """MariaDB+aiomysql sometimes returns DateTime(timezone=True) as a
+    naive datetime. The helper must normalise rather than raising
+    'can't compare offset-naive and offset-aware'."""
+    naive_future = (_utcnow() + timedelta(days=5)).replace(tzinfo=None)
+    st = paid_status(_make_user(credit_until=naive_future))
+    assert st.active is True
+    assert st.source == "credit"
+
+
+# ---------------------------------------------------------------------------
+# is_paid_active — sugar + admin bypass
+# ---------------------------------------------------------------------------
+
+
+def test_is_paid_active_admin_bearer_bypass():
+    """Admin bearer-token (is_admin=True, user=None) always passes — the
+    dev/CLI path must not be artificially gated."""
+    principal = CurrentUser(is_admin=True, user=None)
+    assert is_paid_active(principal) is True
+
+
+def test_is_paid_active_free_user_principal():
+    principal = CurrentUser(is_admin=False, user=_make_user(tier="free"))
+    assert is_paid_active(principal) is False
+
+
+def test_is_paid_active_paid_user_principal():
+    principal = CurrentUser(is_admin=False, user=_make_user(tier="paid"))
+    assert is_paid_active(principal) is True
+
+
+def test_is_paid_active_accepts_bare_user():
+    """Sugar: accepts a User row directly, not just a CurrentUser."""
+    assert is_paid_active(_make_user(tier="paid")) is True
+    assert is_paid_active(_make_user(tier="free")) is False
+
+
+def test_is_paid_active_none():
+    assert is_paid_active(None) is False