Artwork panels: how full, the hit rate and evictions (#198)

Kept bytes against the limit was a flat line once the cache filled. Now a
percentage of the limit beside the share served from disk, and the images each
sweep evicts. The counter shipped minutes ago as ipx_art_trimmed_total is
renamed ipx_art_evicted_total before anything came to read it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/art.rs

@@ -98,7 +98,7 @@ pub fn trim(dir: &Path, limit: u64) -> usize {
         }
     }
     crate::metrics::STATS.art_cache_bytes.store(total, std::sync::atomic::Ordering::Relaxed);
-    crate::metrics::STATS.art_trimmed.fetch_add(gone as u64, std::sync::atomic::Ordering::Relaxed);
+    crate::metrics::STATS.art_evicted.fetch_add(gone as u64, std::sync::atomic::Ordering::Relaxed);
     gone
 }