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>

nostrdb/UnownedNdbNote.swift

@@ -150,6 +150,7 @@ struct UnownedNdbNote: ~Copyable {
     var createdAt: UInt32 { _ndbNote.created_at }
     var id: NoteId { _ndbNote.id }
     var sig: Signature { _ndbNote.sig }
+    var referenced_pubkeys: References<Pubkey> { _ndbNote.referenced_pubkeys }
     
     func toOwned() -> NdbNote {
         return _ndbNote.to_owned()