feat: chat — edit/delete messages + emoji reactions

- Migration 074: edited_at, deleted_at columns + chat_reactions table
- Backend: PATCH edit (own only), DELETE soft-delete (own/admin), POST toggle reaction
  fetchMessage helper returns full message with aggregated reactions in one query
- API types: ChatReaction interface, editedAt/deletedAt/reactions in ChatMessage
- ChatWidget: hover toolbar (😊 react · ✏️ edit · 🗑️ delete), inline edit with
  save/cancel (Enter/Esc), reaction emoji picker (8 emojis), reaction pills below
  messages (click to toggle), '· изм.' label on edited messages, soft-deleted
  messages show 'Сообщение удалено' placeholder

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/074_chat_edit_delete_reactions.sql

@@ -0,0 +1,11 @@
+ALTER TABLE chat_messages ADD COLUMN IF NOT EXISTS edited_at  TIMESTAMPTZ;
+ALTER TABLE chat_messages ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+
+CREATE TABLE IF NOT EXISTS chat_reactions (
+  message_id UUID        NOT NULL REFERENCES chat_messages(id) ON DELETE CASCADE,
+  user_id    UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  emoji      VARCHAR(12) NOT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  PRIMARY KEY (message_id, user_id, emoji)
+);
+CREATE INDEX IF NOT EXISTS idx_chat_reactions_message ON chat_reactions(message_id);