feat(chat): service-level sensitive gating + local sensitive marks

Wires the sensitive-conversation plumbing into ChatService + Settings:
- Settings gains a persisted local sensitive set (isChatSensitive /
  setChatSensitive / sensitiveChatConversations), mirroring the blocked set, for
  INSTANT effect the moment the user marks a conversation (before the on-chain
  signal is harvested back).
- ChatService::isConversationSensitive() OR-combines that local mark with the
  on-chain reduction, and setPersistence() wires it as the DB's per-row key
  selector (sensitive rows -> vault key).
- ingest() withholds a sensitive conversation's messages while the vault key is
  absent (fail-closed: not shown, not persisted); they re-import + persist under
  the vault key once it is unlocked and a scan runs. loadFromDatabase() already
  withholds via the DB's two-key try.
Service-tested end to end against a real ChatDatabase (ingest + reload).

Phase B, step 4 (service gating). App vault lifecycle + UI next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/chat/chat_service.h

@@ -53,7 +53,8 @@ public:
 
     // Attach a persistent backing store (Phase 2). Not owned. Pass nullptr to detach. New messages
     // from ingest() are written through; loadFromDatabase() rehydrates the in-memory store from it.
-    void setPersistence(ChatDatabase* db) { db_ = db; }
+    // Also wires the DB's per-conversation sensitivity predicate to isConversationSensitive().
+    void setPersistence(ChatDatabase* db);
     // Load previously-persisted messages (already decrypted at ingest, re-encrypted at rest under
     // the seed-derived key) into the in-memory store. No-op without an unlocked database.
     void loadFromDatabase();
@@ -89,6 +90,16 @@ public:
     // Conversations whose latest on-chain control op is "sensitive", each paired with its control label.
     std::vector<std::pair<std::string, std::string>> onChainSensitiveConversations() const;
 
+    // A local "sensitive" predicate (typically Settings::isChatSensitive) giving INSTANT effect the moment
+    // the user marks a conversation, before the on-chain signal is harvested back. OR-combined with the
+    // on-chain reduction by isConversationSensitive().
+    void setLocalSensitivePredicate(std::function<bool(const std::string&)> pred) {
+        local_sensitive_pred_ = std::move(pred);
+    }
+    // True if this conversation is sensitive (locally marked OR on-chain). Its rows are stored under the
+    // local vault key; while that key is absent its messages are withheld from the store on ingest/load.
+    bool isConversationSensitive(const std::string& conversationId) const;
+
     // --- Outgoing (compose) ---
     // My chat public key (hex), or "" without an identity — goes in an outgoing header's "p".
     std::string identityPublicKeyHex() const;
@@ -131,6 +142,7 @@ private:
     ChatStore store_;
     ChatDatabase* db_ = nullptr;  // optional; not owned
     std::function<bool(const std::string&)> blocked_pred_;  // true => cid is blocked (drop its messages)
+    std::function<bool(const std::string&)> local_sensitive_pred_;  // true => cid locally marked sensitive
 
     // Reduced on-chain control state, cid -> latest signal. Not persisted (re-derived from chain).
     struct ControlState {