fix(security): consume magic-link tokens atomically

Validation and marking-used were two separate steps, so two concurrent
requests for the same registration token could both pass validation before
either marked it used — a replay window.

Add an atomic consume() at the repository (conditional UPDATE ... WHERE
used = 0 AND not expired, gated on rowcount) and service layers, and switch
the /register handler to consume() instead of validate()+mark_used().

Refs: porchlight-ur7

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_store/test_sqlite_magic_link_repo.py

@@ -63,6 +63,34 @@ async def test_mark_used_not_found(magic_link_repo: SQLiteMagicLinkRepository) -
     assert marked is False
 
 
+async def test_consume_marks_used_and_returns_link(magic_link_repo: SQLiteMagicLinkRepository) -> None:
+    await magic_link_repo.create(_make_link())
+
+    consumed = await magic_link_repo.consume("abc123")
+    assert consumed is not None
+    assert consumed.used is True
+    assert consumed.username == "alice"
+
+
+async def test_consume_is_single_use(magic_link_repo: SQLiteMagicLinkRepository) -> None:
+    await magic_link_repo.create(_make_link())
+
+    first = await magic_link_repo.consume("abc123")
+    assert first is not None
+    # A second consume of the same token must fail (atomic single-use).
+    second = await magic_link_repo.consume("abc123")
+    assert second is None
+
+
+async def test_consume_expired_returns_none(magic_link_repo: SQLiteMagicLinkRepository) -> None:
+    await magic_link_repo.create(_make_link(token="exp", expires_at=datetime.now(UTC) - timedelta(hours=1)))
+    assert await magic_link_repo.consume("exp") is None
+
+
+async def test_consume_nonexistent_returns_none(magic_link_repo: SQLiteMagicLinkRepository) -> None:
+    assert await magic_link_repo.consume("nope") is None
+
+
 async def test_delete_expired(magic_link_repo: SQLiteMagicLinkRepository) -> None:
     expired = _make_link(token="expired", expires_at=datetime.now(UTC) - timedelta(hours=1))
     await magic_link_repo.create(expired)