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>

nostrdb/Ndb+Compaction.swift

@@ -0,0 +1,132 @@
+//
+//  Ndb+Compaction.swift
+//  damus
+//
+
+import Foundation
+
+extension Ndb {
+    /// Makes a compacted copy of the database in a separate directory.
+    ///
+    /// This uses `mdb_env_copy2` with `MDB_CP_COMPACT` (flag = 0x01), which omits free pages
+    /// and sequentially renumbers all pages in the output, reducing database file size.
+    /// - Parameter path: The directory path where the compacted database will be written.
+    ///                   The directory must already exist and be empty.
+    func compact(to path: String) throws {
+        enum CompactError: Error {
+            case mdbOperationError(errno: Int32)
+        }
+
+        try withNdb({
+            try path.withCString({ pathCString in
+                let rc = ndb_snapshot(self.ndb.ndb, pathCString, Self.MDB_CP_COMPACT)
+                guard rc == 0 else {
+                    throw CompactError.mdbOperationError(errno: rc)
+                }
+            })
+        })
+    }
+
+    /// LMDB compact-copy flag.  Passed to `ndb_snapshot` / `mdb_env_copy2` to produce a
+    /// compacted (free-page-omitting) database copy.  Mirrors `MDB_CP_COMPACT = 0x01` from lmdb.h.
+    private static let MDB_CP_COMPACT: UInt32 = 1
+
+    /// Name of the temporary subdirectory created during an in-place compaction.
+    private static let compactTempDirName = "ndb_compact_temp"
+
+    /// The `UserDefaults` key used to signal that the database should be compacted on the next app launch.
+    static let compact_on_next_launch_key = "ndb_compact_on_next_launch"
+
+    /// Requests that the database be compacted the next time the app launches.
+    ///
+    /// Call this to schedule a one-time compaction. The flag is cleared automatically after
+    /// a successful compaction in `compact_if_needed()`.
+    static func set_compact_on_next_launch() {
+        UserDefaults.standard.set(true, forKey: compact_on_next_launch_key)
+    }
+
+    /// Compacts the NostrDB database files if the compact-on-next-launch flag is set.
+    ///
+    /// This is intended to be called once during app startup **before** `Ndb` is opened for
+    /// normal use.  The algorithm is:
+    ///   1. Open a temporary `Ndb` instance at the same path to access the LMDB environment.
+    ///   2. Write a compacted copy of the database to a sibling temp directory.
+    ///   3. Close the temporary `Ndb` instance.
+    ///   4. Atomically replace the original `data.mdb` with the compacted copy.
+    ///   5. Remove the temp directory.
+    ///   6. Clear the flag so compaction does not run again on the following launch.
+    ///
+    /// - Parameter db_path: Override the database directory path.  Pass `nil` (default) to use
+    ///   `Ndb.db_path`.  Mainly useful for testing.
+    static func compact_if_needed(db_path: String? = nil) {
+        guard UserDefaults.standard.bool(forKey: compact_on_next_launch_key) else { return }
+
+        guard let path = db_path ?? Self.db_path else {
+            Log.error("compact_if_needed: could not determine db path", for: .storage)
+            return
+        }
+
+        guard db_file_exists(path: path) else {
+            // No database file present yet; nothing to compact — just clear the flag.
+            UserDefaults.standard.set(false, forKey: compact_on_next_launch_key)
+            return
+        }
+
+        Log.info("Compacting NostrDB on startup…", for: .storage)
+
+        let tempPath = "\(path)/\(compactTempDirName)"
+
+        // Clean up any leftover temp directory from a previously failed attempt.
+        try? FileManager.default.removeItem(atPath: tempPath)
+
+        do {
+            try FileManager.default.createDirectory(atPath: tempPath, withIntermediateDirectories: true)
+        } catch {
+            Log.error("compact_if_needed: failed to create temp dir: %@", for: .storage, String(describing: error))
+            return
+        }
+
+        // Open a temporary Ndb instance just to drive the compaction.
+        guard let tempNdb = Ndb(path: path) else {
+            Log.error("compact_if_needed: failed to open ndb for compaction", for: .storage)
+            try? FileManager.default.removeItem(atPath: tempPath)
+            return
+        }
+        // Ensure the temporary Ndb is closed regardless of how this function exits.
+        defer { tempNdb.close() }
+
+        do {
+            try tempNdb.compact(to: tempPath)
+        } catch {
+            Log.error("compact_if_needed: compaction failed: %@", for: .storage, String(describing: error))
+            try? FileManager.default.removeItem(atPath: tempPath)
+            return
+        }
+        
+        tempNdb.close()
+
+        // Atomically replace the original data.mdb with the compacted copy.
+        let originalDataMdb = URL(fileURLWithPath: "\(path)/\(main_db_file_name)")
+        let compactedDataMdb = URL(fileURLWithPath: "\(tempPath)/\(main_db_file_name)")
+
+        do {
+            _ = try FileManager.default.replaceItemAt(
+                originalDataMdb,
+                withItemAt: compactedDataMdb,
+                backupItemName: nil,
+                options: [.usingNewMetadataOnly]
+            )
+            Log.info("NostrDB compacted successfully", for: .storage)
+        } catch {
+            Log.error("compact_if_needed: failed to replace db file: %@", for: .storage, String(describing: error))
+            try? FileManager.default.removeItem(atPath: tempPath)
+            return
+        }
+
+        // Clean up the temp directory (any remaining files such as lock.mdb).
+        try? FileManager.default.removeItem(atPath: tempPath)
+
+        // Clear the flag so we don't compact again on the next launch.
+        UserDefaults.standard.set(false, forKey: compact_on_next_launch_key)
+    }
+}