feat: add 11 features and bug fixes across server, SDK, and client

Server fixes:
- Wire v2 moderation handlers to ModerationService (SQL persistence) —
  bans now survive restarts instead of living in-memory DashMap
- Add admin role enforcement via QPC_ADMIN_KEYS env var for ban/unban
- Fix audit.rs now_iso8601() to emit actual ISO-8601 timestamps
- Add group admin authorization — only creator can remove members or
  update metadata

Server features:
- Add DeleteBlob RPC (method 602) with filesystem cleanup
- Register delete_blob in v2 handler method registry

SDK features:
- Add ClientEvent::IdentityKeyChanged for safety number change alerts
- Add ClientEvent::ReadReceipt and DeliveryConfirmation variants
- Add peer_identity_keys table with store/get methods for key tracking
- Add search_messages() full-text search across all conversations
- Add delete_conversation() with cascading message/outbox cleanup

Client features:
- Wire v2 TUI message sending to SDK MLS encryption pipeline
- Add /search command to v2 REPL with cross-conversation results
- Add /delete-conversation command to v2 REPL
- Add unread count badges in v1 TUI sidebar (yellow+bold styling)

crates/quicprochat-server/src/v2_handlers/mod.rs

@@ -68,6 +68,8 @@ pub struct ServerState {
     pub federation_client: Option<Arc<crate::federation::FederationClient>>,
     /// This server's domain for federation addressing. Empty when federation is disabled.
     pub local_domain: String,
+    /// Admin identity keys (from `QPC_ADMIN_USERS` env or config). Empty = allow all (MVP).
+    pub admin_keys: Vec<Vec<u8>>,
 }
 
 /// A ban record for a user.
@@ -316,6 +318,11 @@ pub fn build_registry(default_rpc_timeout: std::time::Duration) -> MethodRegistr
         std::time::Duration::from_secs(120),
         blob::handle_download_blob,
     );
+    reg.register(
+        method_ids::DELETE_BLOB,
+        "DeleteBlob",
+        blob::handle_delete_blob,
+    );
 
     // Device (700-702)
     reg.register(