Add EntityPreloader for batched profile metadata preloading

Implements an actor-based preloading system to efficiently fetch profile
metadata for note authors and referenced users. The EntityPreloader queues
requests and batches them intelligently (500 pubkeys or 1 second timeout)
to avoid network overload while improving UX by ensuring profiles are
available when rendering notes.

Key changes:
- Add EntityPreloader actor with queue-based batching logic
- Integrate with SubscriptionManager via PreloadStrategy enum
- Add lifecycle management (start/stop on app foreground/background)
- Skip preload for pubkeys already cached in ndb
- Include comprehensive test suite with 11 test cases covering batching,
  deduplication, and edge cases
- Optimize ProfilePicView to load from ndb before first render

Closes: https://github.com/damus-io/damus/issues/gh-3511
Changelog-Added: Profile metadata preloading for improved timeline performance
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/Features/Search/Models/SearchHomeModel.swift

@@ -62,7 +62,7 @@ class SearchHomeModel: ObservableObject {
         var follow_list_filter = NostrFilter(kinds: [.follow_list])
         follow_list_filter.until = UInt32(Date.now.timeIntervalSince1970)
         
-        for await item in damus_state.nostrNetwork.reader.advancedStream(filters: [get_base_filter(), follow_list_filter], to: to_relays) {
+        for await item in damus_state.nostrNetwork.reader.advancedStream(filters: [get_base_filter(), follow_list_filter], to: to_relays, preloadStrategy: .preload) {
             switch item {
             case .event(lender: let lender):
                 await lender.justUseACopy({ event in