Create NostrDB streaming and async lookup interfaces

This commit introduces new interfaces for working with NostrDB from
Swift, including `NostrFilter` conversion, subscription streaming via
AsyncStreams and lookup/wait functions.

No user-facing changes.

Changelog-None
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

nostrdb/Ndb+.swift

@@ -0,0 +1,30 @@
+//
+//  Ndb+.swift
+//  damus
+//
+//  Created by Daniel D’Aquino on 2025-04-04.
+//
+
+/// ## Implementation notes
+///
+/// 1. This was created as a separate file because it contains dependencies to damus-specific structures such as `NostrFilter`, which is not yet available inside the NostrDB codebase.
+
+import Foundation
+
+extension Ndb {
+    /// Subscribe to events matching the provided NostrFilters
+    /// - Parameters:
+    ///   - filters: Array of NostrFilter objects
+    ///   - maxSimultaneousResults: Maximum number of initial results to return
+    /// - Returns: AsyncStream of StreamItem events
+    /// - Throws: NdbStreamError if subscription fails
+    func subscribe(filters: [NostrFilter], maxSimultaneousResults: Int = 1000) throws(NdbStreamError) -> AsyncStream<StreamItem> {
+        let ndbFilters: [NdbFilter]
+        do {
+            ndbFilters = try filters.toNdbFilters()
+        } catch {
+            throw .cannotConvertFilter(error)
+        }
+        return try self.subscribe(filters: ndbFilters, maxSimultaneousResults: maxSimultaneousResults)
+    }
+}