Graph the artwork thrown out, not the artwork kept (#198)

The sweep trims the cache to its limit, so the kept bytes it reports are the
limit every time once the cache has filled: the panel was two lines flat at
500 MiB. A counter of images thrown out says whether the limit is too small.

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

src/art.rs

@@ -98,6 +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);
     gone
 }
 

src/metrics.rs

@@ -30,6 +30,9 @@ pub struct Stats {
     /// When DeadAir started on the file it is on, Unix seconds; 0 when it is on none.
     pub deadair_busy_since: AtomicI64,
     pub art_cache_bytes: AtomicU64,
+    /// Images the sweep threw out to keep under the limit: kept bytes sit at the limit by design,
+    /// so this is what says whether the limit is too small.
+    pub art_trimmed: AtomicU64,
     /// The library as the healthcheck last counted it, every 30 s; -1 until it has.
     pub feeds: AtomicI64,
     pub pending: AtomicI64,
@@ -52,6 +55,7 @@ impl Stats {
             deadair_backlog: AtomicI64::new(0),
             deadair_busy_since: AtomicI64::new(0),
             art_cache_bytes: AtomicU64::new(0),
+            art_trimmed: AtomicU64::new(0),
             feeds: AtomicI64::new(-1),
             pending: AtomicI64::new(-1),
             downloaded: AtomicI64::new(-1),
@@ -205,6 +209,7 @@ fn work(out: &mut String, st: &Stats, event_queue: usize, retries: usize, art_li
     gauge(out, "ipx_feed_retries", "Feeds waiting to try a failed name lookup again.", &[("", retries as f64)]);
     gauge(out, "ipx_art_cache_bytes", "Artwork kept on disk, as of the last sweep.", &[("", st.art_cache_bytes.load(Relaxed) as f64)]);
     gauge(out, "ipx_art_cache_limit_bytes", "The most artwork kept.", &[("", art_limit as f64)]);
+    counter(out, "ipx_art_trimmed_total", "Images thrown out to keep the artwork under its limit.", &[("", st.art_trimmed.load(Relaxed) as f64)]);
     for (name, help, v) in [
         ("ipx_feeds", "Feeds in the catalogue, as the healthcheck last counted.", &st.feeds),
         ("ipx_downloads_pending", "Files a scan will download on its own, as the healthcheck last counted.", &st.pending),