store: remove dead-code branch from ClaimLegacyOwner

Task 3 correction: migration 0003 unconditionally seeds a profile row on every
fresh install, and Task 1's migration 0023 preserves this seeded row with
user_id = NULL. Therefore, a fresh install always has at least one profile row
with user_id IS NULL when users table is empty -- the "no-op on genuinely fresh
install" scenario was unreachable dead code. Confirmed with the codebase owner
that no scenario requires defending against a missing profile row.

Collapse the two-branch error handling into a single `if err != nil` check
(matching the pattern used elsewhere in the function), remove the now-unused
database/sql import, update the function's doc comment to remove the false claim
about fresh installs, and delete the now-unreachable
TestClaimLegacyOwner_NoOpOnGenuinelyFreshInstall test.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/store/legacy_claim_test.go

@@ -95,15 +95,3 @@ func TestClaimLegacyOwner_NoOpOnceAUserAlreadyExists(t *testing.T) {
 		t.Fatalf("expected exactly the 1 pre-existing user to remain, got %+v", users)
 	}
 }
-
-func TestClaimLegacyOwner_NoOpOnGenuinelyFreshInstall(t *testing.T) {
-	db := openTestDB(t)
-	ctx := context.Background()
-
-	if err := db.ClaimLegacyOwner(ctx, "kriss-sub"); err != nil {
-		t.Fatalf("ClaimLegacyOwner on fresh install: %v", err)
-	}
-	if _, found, err := db.GetUserBySub(ctx, "kriss-sub"); err != nil || found {
-		t.Fatalf("expected no user created on a fresh install with no legacy profile, found=%v err=%v", found, err)
-	}
-}