core: the column holds entropy, so name it entropy

The mnemonic comes back from the entropy exactly, which is what export
returns, and the seed PBKDF2 derives from the mnemonic is computed when a
key is needed and never stored. The column was the one thing called seed
that is not the seed, which is why it needed a comment saying so.

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

src/Simplex/Chat/Store/Postgres/Migrations/chat_schema.sql

@@ -1535,7 +1535,7 @@ ALTER TABLE test_chat_schema.users ALTER COLUMN user_id ADD GENERATED ALWAYS AS
 
 CREATE TABLE test_chat_schema.wallet_seeds (
     wallet_seed_id bigint NOT NULL,
-    seed bytea NOT NULL,
+    entropy bytea NOT NULL,
     next_name_index bigint DEFAULT 1 NOT NULL,
     single_seed smallint DEFAULT 1 NOT NULL
 );