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)

proto/qpc/v1/blob.proto

@@ -27,3 +27,12 @@ message DownloadBlobResponse {
   uint64 total_size = 2;
   string mime_type = 3;
 }
+
+// Method ID: 602
+message DeleteBlobRequest {
+  bytes blob_id = 1;
+}
+
+message DeleteBlobResponse {
+  bool deleted = 1;
+}