fix: add @MainActor to MessageDeduplicationService for thread safety

Added @MainActor annotation to both LRUDeduplicationCache and
MessageDeduplicationService classes. This provides compile-time
enforcement of thread safety since all callers (ChatViewModel,
NostrRelayManager) are already on MainActor.

Benefits:
- Compile-time enforcement prevents future misuse
- Simpler than internal locking mechanisms
- Consistent with existing patterns in codebase

Also adds:
- @MainActor annotation to existing test suites
- 5 new concurrency tests verifying thread safety behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

bitchat/Services/MessageDeduplicationService.swift

@@ -12,6 +12,8 @@ import Foundation
 
 /// Generic LRU (Least Recently Used) cache for deduplication.
 /// Uses an efficient O(1) lookup with periodic compaction.
+/// Thread-safe via @MainActor - all callers are already on main actor.
+@MainActor
 final class LRUDeduplicationCache<Value> {
     private var map: [String: Value] = [:]
     private var order: [String] = []
@@ -157,6 +159,8 @@ enum ContentNormalizer {
 
 /// Service that manages message deduplication using LRU caches.
 /// Provides separate caches for content-based dedup and Nostr event ID dedup.
+/// Thread-safe via @MainActor - all callers are already on main actor.
+@MainActor
 final class MessageDeduplicationService {
 
     /// Cache for content-based near-duplicate detection