feat: support optional user_id ticket identifier

Add an alternative ticket identifier scheme: instead of (name, email),
external integrations can issue tickets bound to an LNbits user_id.

- m007 adds the user_id column on events.ticket
- CreateTicket validator enforces exactly one identifier scheme per ticket
- Ticket / PublicTicket: name, email, user_id all Optional
- _parse_ticket_row reverses the empty-string sentinel used to keep the
  NOT NULL name/email columns satisfied when user_id is the identifier
- POST /tickets/{event_id} dispatches to _create_user_id_ticket vs
  _create_named_ticket based on the supplied identifier
- New GET /tickets/user/{user_id} returns tickets for a given user

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations.py

@@ -189,3 +189,14 @@ async def m006_add_extra_fields(db):
     )
     await _alter_add_column_safe(db, "ALTER TABLE events.events ADD COLUMN extra TEXT")
     await _alter_add_column_safe(db, "ALTER TABLE events.ticket ADD COLUMN extra TEXT")
+
+
+async def m007_add_user_id_support(db):
+    """
+    Add user_id column to ticket table so a ticket can reference an LNbits
+    user id instead of (name, email). Application logic enforces that exactly
+    one identifier scheme is used per ticket.
+    """
+    await _alter_add_column_safe(
+        db, "ALTER TABLE events.ticket ADD COLUMN user_id TEXT"
+    )