feat(core): add Item::prune_history honoring retention policy

Forever, LastN, and Days policies all covered. Tests verify drop order
(keeps newest), days cutoff, and forever-no-op semantics.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/idfoto-core/src/item.rs

@@ -215,6 +215,26 @@ impl Item {
     pub fn is_trashed(&self) -> bool {
         self.trashed_at.is_some()
     }
+
+    pub fn prune_history(&mut self, retention: &crate::settings::HistoryRetention, now: i64) {
+        use crate::settings::HistoryRetention;
+        for history in self.field_history.values_mut() {
+            match retention {
+                HistoryRetention::Forever => {}
+                HistoryRetention::LastN(n) => {
+                    let n = *n as usize;
+                    if history.len() > n {
+                        let drop_count = history.len() - n;
+                        history.drain(..drop_count);
+                    }
+                }
+                HistoryRetention::Days(d) => {
+                    let cutoff = now - (*d as i64) * 86_400;
+                    history.retain(|e| e.replaced_at > cutoff);
+                }
+            }
+        }
+    }
 }
 
 /// Serialize a FieldValue to the string form stored in field_history.
@@ -418,4 +438,60 @@ mod tests {
             other => panic!("expected Login, got {:?}", other),
         }
     }
+
+    #[test]
+    fn prune_history_keeps_last_n() {
+        use crate::settings::HistoryRetention;
+
+        let core = ItemCore::Login(crate::item_types::LoginCore::default());
+        let mut item = Item::new("x".into(), core);
+        let f = Field::new("p".into(), FieldValue::Password(Zeroizing::new("v0".into())));
+        let fid = f.id.clone();
+        item.sections.push(Section { name: None, fields: vec![f] });
+
+        for i in 1..=5 {
+            item.set_field_value(&fid, FieldValue::Password(Zeroizing::new(format!("v{i}").into()))).unwrap();
+        }
+        assert_eq!(item.field_history[&fid].len(), 5);
+
+        item.prune_history(&HistoryRetention::LastN(3), 0);
+        assert_eq!(item.field_history[&fid].len(), 3);
+        // Keeps the MOST RECENT 3
+        assert_eq!(item.field_history[&fid][0].value.as_str(), "v2");
+    }
+
+    #[test]
+    fn prune_history_drops_old_entries_by_days() {
+        use crate::settings::HistoryRetention;
+
+        let core = ItemCore::Login(crate::item_types::LoginCore::default());
+        let mut item = Item::new("x".into(), core);
+        let f = Field::new("p".into(), FieldValue::Password(Zeroizing::new("v0".into())));
+        let fid = f.id.clone();
+        item.sections.push(Section { name: None, fields: vec![f] });
+
+        let now = 1_000_000_000;
+        item.field_history.insert(fid.clone(), vec![
+            FieldHistoryEntry { value: Zeroizing::new("old".into()),    replaced_at: now - 100 * 86_400 },
+            FieldHistoryEntry { value: Zeroizing::new("recent".into()), replaced_at: now -   1 * 86_400 },
+        ]);
+
+        item.prune_history(&HistoryRetention::Days(30), now);
+        assert_eq!(item.field_history[&fid].len(), 1);
+        assert_eq!(item.field_history[&fid][0].value.as_str(), "recent");
+    }
+
+    #[test]
+    fn prune_history_forever_keeps_all() {
+        use crate::settings::HistoryRetention;
+
+        let core = ItemCore::Login(crate::item_types::LoginCore::default());
+        let mut item = Item::new("x".into(), core);
+        item.field_history.insert(FieldId::new(), vec![
+            FieldHistoryEntry { value: Zeroizing::new("a".into()), replaced_at: 0 },
+            FieldHistoryEntry { value: Zeroizing::new("b".into()), replaced_at: 0 },
+        ]);
+        item.prune_history(&HistoryRetention::Forever, 1_000_000_000);
+        assert_eq!(item.field_history.values().next().unwrap().len(), 2);
+    }
 }