fix(prop-grid-rs): evict expired IdxCache entries on get

The in-memory idx cache kept entries around forever: get() returned
None on expiry but left the HashMap entry in place. Each hourly
HRRR cycle adds ~38 new idx URLs (19 forecast hours × surface +
pressure), so the map grew ~4 MiB/day indefinitely on each Rust
replica. Not the main cause of the OOM loop (glibc fragmentation
was; jemalloc landed last commit) but worth sealing so a long-
lived pod's idx cache can't drift toward the 3 GiB ceiling over
weeks.

Evicts on-touch via `get()` — simple, amortized, no background
sweep thread needed.

rust/prop_grid_rs/src/fetcher.rs

@@ -294,11 +294,14 @@ impl IdxCache {
     }
 
     pub fn get(&self, url: &str) -> Option<String> {
-        if let Ok(map) = self.inner.lock() {
+        if let Ok(mut map) = self.inner.lock() {
             if let Some(entry) = map.get(url) {
                 if entry.inserted.elapsed() < IDX_TTL {
                     return Some(entry.body.clone());
                 }
+                // Expired entry — evict so unique idx URLs don't
+                // accumulate across hourly HRRR cycles.
+                map.remove(url);
             }
         }
         if let Some(disk) = &self.disk {
@@ -332,6 +335,24 @@ impl IdxCache {
             );
         }
     }
+
+    #[cfg(test)]
+    fn len(&self) -> usize {
+        self.inner.lock().map(|m| m.len()).unwrap_or(0)
+    }
+
+    #[cfg(test)]
+    fn put_with_age(&self, url: String, body: String, age: Duration) {
+        if let Ok(mut map) = self.inner.lock() {
+            map.insert(
+                url,
+                IdxCacheEntry {
+                    body,
+                    inserted: Instant::now() - age,
+                },
+            );
+        }
+    }
 }
 
 // ── HTTP client ──────────────────────────────────────────────────────
@@ -722,6 +743,21 @@ mod tests {
         assert!(cache.get("missing").is_none());
     }
 
+    #[test]
+    fn idx_cache_removes_expired_entries_on_get() {
+        // Without eviction the HashMap grows forever as unique idx URLs
+        // accumulate each hour. Calling get() on an expired entry must
+        // remove it so the in-memory map bounds itself to the hot set.
+        let cache = IdxCache::new();
+        cache.put_with_age("stale".into(), "old".into(), IDX_TTL + Duration::from_secs(1));
+        cache.put("fresh".into(), "new".into());
+
+        assert_eq!(cache.len(), 2);
+        assert!(cache.get("stale").is_none());
+        assert_eq!(cache.len(), 1, "expired entry should have been evicted");
+        assert_eq!(cache.get("fresh").as_deref(), Some("new"));
+    }
+
     #[test]
     fn disk_idx_cache_round_trips() {
         let tmp = tempfile::TempDir::new().unwrap();