feat(provision): capitalize the stall owner name

Before this commit, a username of "greg" produced the stall "greg's Store".
Now it produces "Greg's Store". The change is conservative:
`username[:1].upper() + username[1:]` preserves the existing case of
characters past the first (so "JohnDoe" stays "JohnDoe", not Python's
`.capitalize()` outcome "Johndoe").

Lives in `provision_merchant` so both callers — nostrmarket's lazy
`_auto_create_merchant` and the lnbits-side eager hook
(`_create_default_merchant` per aiolabs/lnbits#9) — benefit from a
single source of truth without each caller having to remember the
formatting convention.

Doesn't touch `merchant.config.display_name` (still defaults to None);
only the stall name string is affected.

services.py

@@ -275,10 +275,11 @@ async def provision_merchant(
     )
     await create_zone(merchant.id, online_zone)
 
-    name = display_name or "My"
+    raw_owner_name = display_name or "My"
+    owner_name = raw_owner_name[:1].upper() + raw_owner_name[1:]
     default_stall = Stall(
         wallet=wallet_id,
-        name=f"{name}'s Store",
+        name=f"{owner_name}'s Store",
         currency="sat",
         shipping_zones=[online_zone],
     )