Mechanism of Friedel-Crafts acylation with succinic anhydride
The reactivity of acid chlorides and acid anhydrides are actually quite similar. It may be helpful for you to think of an acid anhydride like this:

When we do, the acylation reaction is almost identical to that of an acid chloride, where an acylium cation $\ce{R-C#O+}$ is generated:

The subsequent proton exchange restores aromaticity: