gleam: update README with Gleam implementation

[HeyItWorked]

Task

Update the root README.md to include the Gleam implementation.

Changes needed

1. Add gleam-bookshelf/ to the architecture tree
2. Add Gleam row to the URL table: Gleam | http://localhost:8082
3. Add testing instructions for Gleam:
   ```bash
   cd gleam-bookshelf
   DATABASE_URL="postgres://shelf:shelf@db:5432/bookshelf?sslmode=disable" gleam test
   ```
4. Update implementations checklist:
   • Gleam (Wisp + gleam_pgo)

Acceptance criteria

• README accurately reflects all three implementations
• Port 8082 documented