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>

justfile

@@ -1,5 +1,10 @@
 build:
     xcodebuild -scheme damus -sdk iphoneos -destination 'platform=iOS Simulator,OS=26.0,name=iPhone 16e' -quiet | xcbeautify --quieter
 
-test:
-    xcodebuild test -scheme damus -destination 'platform=iOS Simulator,OS=26.0,name=iPhone 16e' | xcbeautify
+test test_name="":
+    #!/usr/bin/env bash
+    if [ -n "{{test_name}}" ]; then
+        xcodebuild test -scheme damus -destination 'platform=iOS Simulator,OS=26.0,name=iPhone 16e' -only-testing:{{test_name}} | xcbeautify --quieter
+    else
+        xcodebuild test -scheme damus -destination 'platform=iOS Simulator,OS=26.0,name=iPhone 16e' | xcbeautify --quieter
+    fi