feat: let a user_id ticket carry an email, accept frontend_url

`CreateTicket` no longer rejects `user_id` together with `name`/`email`
(the exclusion was a fork-only dispatch convenience from dfabcb8; nothing
needed it). `crud.create_ticket` stops blanking name/email when a user_id
is present, so logged-in webapp buyers can have their ticket emailed —
until now `_send_ticket_notification` short-circuited on the empty
address for every app purchase.

New optional `frontend_url` (absolute http(s) root, no query/fragment/..,
trailing slash stripped) lets a buyer-side client name the app the buyer
should be returned to and linked into from the ticket email; the origin
allow-list lives in views_api.

Also folds in the pending black reflow of migrations_fork.py.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EYwoAkZZmXMMmaBp4WGUBo

tests/test_crud_ticket_email.py

@@ -0,0 +1,48 @@
+from unittest.mock import AsyncMock
+
+import pytest
+
+from .. import crud
+
+
+@pytest.mark.asyncio
+async def test_create_ticket_keeps_email_alongside_user_id(monkeypatch):
+    inserted = {}
+
+    async def fake_insert(table, model):
+        inserted["table"] = table
+        inserted["model"] = model
+
+    monkeypatch.setattr(crud.db, "insert", AsyncMock(side_effect=fake_insert))
+
+    ticket = await crud.create_ticket(
+        payment_hash="hash",
+        wallet="w",
+        event="e",
+        name="Ada",
+        email="ada@example.com",
+        user_id="u1",
+        ticket_id="t1",
+    )
+
+    assert inserted["table"] == "events.ticket"
+    assert inserted["model"].user_id == "u1"
+    assert inserted["model"].email == "ada@example.com"
+    assert inserted["model"].name == "Ada"
+    assert ticket.email == "ada@example.com"
+
+
+@pytest.mark.asyncio
+async def test_create_ticket_stores_empty_string_sentinels(monkeypatch):
+    inserted = {}
+
+    async def fake_insert(table, model):
+        inserted["model"] = model
+
+    monkeypatch.setattr(crud.db, "insert", AsyncMock(side_effect=fake_insert))
+
+    await crud.create_ticket(
+        payment_hash="hash", wallet="w", event="e", user_id="u1", ticket_id="t2"
+    )
+    assert inserted["model"].email == ""
+    assert inserted["model"].name == ""