core: name the unique index on wallet_seeds

The one key per device rule is meant to be lifted later. SQLite cannot drop
a column with an inline UNIQUE, or its automatic index, so lifting it would
have meant rebuilding the table. As a named index it is a DROP INDEX and a
DROP COLUMN.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rvc3HbiWBTqbAvRT45G5oX

src/Simplex/Chat/Wallet.hs

@@ -2,8 +2,8 @@
 
 -- | BIP-39 seeds and the keys derived from them.
 --
--- One key per name, not one per profile, so that exporting a name's key hands
--- over that name only.
+-- Two layers: one account path per profile, and one key per name under it. A
+-- per-profile key would hand over every name that profile owns.
 module Simplex.Chat.Wallet
   ( SeedId (..),
     WalletSeed (..),