feat: add configurable max hashtag limit filter for spam prevention

Adds a security setting to hide posts with more than X hashtags (t tags),
defaulting to 5. Applied at the feed filter level via FilterByListParams.match()
and Account.isAcceptable() to cover all feeds. Configurable in Security Filters
settings, with 0 to disable.

https://claude.ai/code/session_01NMVypgGSU9EjQA7hZ9uvjD

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/model/IAccount.kt

@@ -59,6 +59,7 @@ data class LiveHiddenUsers(
     val hiddenUsers: Set<String> = emptySet(),
     val spammers: Set<String> = emptySet(),
     val hiddenWords: Set<String> = emptySet(),
+    val maxHashtagLimit: Int = 5,
 ) {
     fun isUserHidden(userHex: String) = hiddenUsers.contains(userHex) || spammers.contains(userHex)
 }