feat(chat): per-conversation delete (revive + block); memoize chat/badge render

Adds a per-conversation "delete" with two modes, and removes the per-frame
rescans of the chat history that shared these files.

Delete conversation (header trash icon → confirm dialog):
- Delete (revive-on-new-message): clears local history and tombstones the
  messages (new chat_deleted table, keyed dedup hashes) so the every-few-
  seconds memo re-scan can't re-import them; a genuinely NEW message (new
  txid) revives the thread.
- Delete & block: removes history WITHOUT a tombstone and records the cid as
  blocked (settings); ChatService::ingest drops that conversation's messages
  — old and future — until unblocked from the "Blocked" manager, which then
  re-imports the conversation from chain.
- Local-only (messages remain on-chain; the peer keeps their copy).
  deleteConversation() deletes the DB rows FIRST and only then mutates the
  store, so a failed write can't leave the two diverged.
Unit-tested (revive / tombstone-survives-reload / block / unblock) and
adversarially reviewed (store/DB divergence, half-open DB, revive-unread).

Performance (chat + badge hot paths, from the perf audit):
- ChatStore gains revision(); the Chat unread badge is now a single O(N)
  no-alloc pass cached on it (was O(conversations x messages) copy+sort every
  frame). The conversation list and open thread are memoized on revision()
  (+ show-hidden and AddressBook::revision() for peer-name resolution).
- AddressBook gains revision() so an in-place contact rename invalidates the
  chat memo (an edit keeps entries().size() constant).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/chat/chat_service.h

@@ -11,6 +11,7 @@
 #include "chat_store.h"
 
 #include <cstdint>
+#include <functional>
 #include <string>
 #include <unordered_map>
 #include <vector>
@@ -55,6 +56,19 @@ public:
     // the seed-derived key) into the in-memory store. No-op without an unlocked database.
     void loadFromDatabase();
 
+    // Set a predicate returning true for a BLOCKED conversation id. ingest() drops those messages
+    // entirely (they are never stored or persisted) until the predicate stops returning true — this is
+    // how "delete & block" suppresses old and future messages. Typically wired to Settings::isChatBlocked.
+    void setBlockedPredicate(std::function<bool(const std::string&)> pred) { blocked_pred_ = std::move(pred); }
+
+    // Locally delete a conversation: remove its messages from the store and the database. When `block`
+    // is false, the removed messages are tombstoned so a chain re-scan won't re-import them, but a
+    // genuinely NEW message (new txid) revives the thread. When `block` is true, the rows are removed
+    // WITHOUT a tombstone (the caller also records the cid as blocked via the predicate above); unblocking
+    // later lets the conversation re-import from chain. Returns false (leaving the store untouched) if the
+    // persisted rows couldn't be removed, so the caller can avoid a store/DB divergence.
+    bool deleteConversation(const std::string& conversationId, bool block);
+
     // --- Outgoing (compose) ---
     // My chat public key (hex), or "" without an identity — goes in an outgoing header's "p".
     std::string identityPublicKeyHex() const;
@@ -90,6 +104,7 @@ private:
     bool has_identity_ = false;
     ChatStore store_;
     ChatDatabase* db_ = nullptr;  // optional; not owned
+    std::function<bool(const std::string&)> blocked_pred_;  // true => cid is blocked (drop its messages)
 };
 
 } // namespace dragonx::chat