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

crud.py

@@ -55,14 +55,12 @@ async def create_ticket(
     now = datetime.now(timezone.utc)
     row_id = ticket_id or payment_hash
 
-    # name/email columns are NOT NULL in the schema, so we store "" when only
-    # user_id is supplied. _parse_ticket_row reverses this on read.
-    if user_id:
-        db_name = ""
-        db_email = ""
-    else:
-        db_name = name or ""
-        db_email = email or ""
+    # name/email columns are NOT NULL in the schema, so we store "" when a
+    # value is absent. _parse_ticket_row reverses this on read. A user_id
+    # ticket may carry an email too — that is how logged-in webapp buyers get
+    # their ticket emailed.
+    db_name = name or ""
+    db_email = email or ""
 
     db_ticket = Ticket(
         id=row_id,