feat: implement LMDB compact solution for storage optimization

- Add `compact(to:)` method on `Ndb` using `mdb_env_copy2` / `ndb_snapshot`
  with `MDB_CP_COMPACT` flag to produce a smaller compacted database copy
- Add `compact_if_needed(db_path:)` static startup method that reads a
  UserDefaults flag, opens a temp Ndb, compacts to a sibling temp dir,
  atomically replaces `data.mdb`, cleans up, and clears the flag
- Add `set_compact_on_next_launch()` to schedule compaction from the UI
- Call `Ndb.compact_if_needed()` in `ContentView.connect()` before the
  main Ndb instance is opened
- Add `CompactDatabaseButton` to `StorageSettingsView` with a confirmation
  dialog that informs the user a restart is needed
- Add `NdbCompactionTests` unit tests covering flag API, no-op path,
  missing-DB path, and full round-trip compaction

Closes: https://github.com/damus-io/damus/issues/3680
Changelog-Added: Added "Compact Database" button in Settings → Storage that reclaims unused space by compacting the NostrDB on the next app launch
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/ContentView.swift

@@ -140,6 +140,18 @@ struct ContentView: View {
     // connect retry timer
     let timer = Timer.publish(every: 1, on: .main, in: .common).autoconnect()
     
+    init(keypair: Keypair, appDelegate: AppDelegate?) {
+        // Compact the database if requested from the previous session.
+        // This runs before opening the main Ndb instance so that it works on an idle database.
+        // This also gets run here instead of `connect` because we should anticipate this to add a few seconds of delay in worst case scenarios.
+        // If we were to add this in the `connect` function, parallel functions that depend on `damus_state!` could cause crashes in the app.
+        // By placing this here, we only delay the splash screen a bit
+        Ndb.compact_if_needed()
+        
+        self.keypair = keypair
+        self.appDelegate = appDelegate
+    }
+    
     func navIsAtRoot() -> Bool {
         return navigationCoordinator.isAtRoot()
     }
@@ -1168,4 +1180,5 @@ func logout(_ state: DamusState?)
 {
     state?.close()
     notify(.logout)
-}
+}
+

damus/Features/Settings/Views/StorageSettingsView.swift

@@ -18,6 +18,12 @@ fileprivate enum CacheClearingState {
     case cleared
 }
 
+/// A simple type to keep track of the compact scheduling state
+fileprivate enum CompactSchedulingState {
+    case not_scheduled
+    case scheduled
+}
+
 /// Storage category for display in list and chart
 struct StorageCategory: Identifiable {
     let id: String
@@ -46,6 +52,8 @@ struct StorageSettingsView: View {
     @State private var isPreparingExport: Bool = false
     @State fileprivate var cache_clearing_state: CacheClearingState = .not_cleared
     @State var showing_cache_clear_alert: Bool = false
+    @State fileprivate var compact_scheduling_state: CompactSchedulingState = .not_scheduled
+    @State var showing_compact_alert: Bool = false
     
     /// Storage categories with cumulative ranges for angle selection (iOS 17+)
     private var categoryRanges: [(category: String, range: Range<Double>)] {
@@ -164,6 +172,7 @@ struct StorageSettingsView: View {
                 // Clear Cache Section
                 Section {
                     self.ClearCacheButton
+                    self.CompactDatabaseButton
                 }
             }
             
@@ -215,6 +224,10 @@ struct StorageSettingsView: View {
             if stats == nil {
                 loadStorageStats()
             }
+            // Reflect any previously scheduled compaction in the button state.
+            if UserDefaults.standard.bool(forKey: Ndb.compact_on_next_launch_key) {
+                compact_scheduling_state = .scheduled
+            }
         }
         .onReceive(handle_notify(.switched_timeline)) { _ in
             dismiss()
@@ -338,6 +351,38 @@ struct StorageSettingsView: View {
                   secondaryButton: .cancel())
         }
     }
+
+    /// Compact database button view with confirmation dialog.
+    ///
+    /// Schedules a one-time database compaction to run on the next app launch.  The user
+    /// is informed that the app will need to restart to complete the operation.
+    var CompactDatabaseButton: some View {
+        Button(action: { self.showing_compact_alert = true }, label: {
+            HStack(spacing: 6) {
+                switch compact_scheduling_state {
+                    case .not_scheduled:
+                        Text("Compact Database", comment: "Button to compact the NostrDB database on next launch.")
+                    case .scheduled:
+                        Image(systemName: "checkmark.circle.fill")
+                            .foregroundColor(.green)
+                            .accessibilityHidden(true)
+                        Text("Compaction scheduled. Restart app to continue.", comment: "Message indicating that a database compaction has been scheduled for the next app launch.")
+                }
+            }
+        })
+        .disabled(self.compact_scheduling_state != .not_scheduled)
+        .alert(isPresented: $showing_compact_alert) {
+            Alert(
+                title: Text("Compact Database", comment: "Confirmation dialog title for database compaction"),
+                message: Text("This will reclaim unused space in the database. The app will need to restart to complete the operation. Proceed?", comment: "Message explaining what database compaction does and that a restart is required."),
+                primaryButton: .default(Text("OK", comment: "Button label indicating user wants to proceed.")) {
+                    Ndb.set_compact_on_next_launch()
+                    compact_scheduling_state = .scheduled
+                },
+                secondaryButton: .cancel()
+            )
+        }
+    }
 }
 
 /// Pie chart displaying storage usage distribution (iOS 17+)