refactor: consolidate blockchain explorer into single app and update backup ignore patterns

- Remove standalone explorer-web app (README, HTML, package files)
- Add /web endpoint to blockchain-explorer for web interface access
- Update .gitignore to exclude application backup archives (*.tar.gz, *.zip)
- Add backup documentation files to .gitignore (BACKUP_INDEX.md, README.md)
- Consolidate explorer functionality into main blockchain-explorer application

docs/6_architecture/8_codebase-structure.md

@@ -91,21 +91,14 @@ apps/coordinator-api/
 └── pyproject.toml
 ```
 
-### explorer-web
-Blockchain explorer SPA built with TypeScript and Vite.
+### blockchain-explorer
+Agent-first blockchain explorer built with Python FastAPI and built-in HTML interface.
 
 ```
-apps/explorer-web/
-├── src/
-│   ├── main.ts              # Application entry
-│   ├── config.ts            # API configuration
-│   ├── components/          # UI components (header, footer, data mode toggle, notifications)
-│   ├── lib/                 # Data models and mock data
-│   └── pages/               # Page views (overview, blocks, transactions, addresses, receipts)
-├── public/                  # Static assets (CSS themes, mock JSON data)
-├── tests/e2e/               # Playwright end-to-end tests
-├── vite.config.ts
-└── tsconfig.json
+apps/blockchain-explorer/
+├── main.py                   # FastAPI application entry
+├── systemd service           # Production service file
+└── EXPLORER_MERGE_SUMMARY.md # Architecture documentation
 ```
 
 ### marketplace-web