Add backend/README.md with Quick Start Guide

backend/README.md

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+# backend - Go Game Server
+
+An authoritative game server for a Towerfall-inspired arena combat game. Handles real-time physics simulation, WebSocket communication, and HTTP APIs for game management and ML bot integration.
+
+For detailed architecture information, see [docs/architecture/backend.md](../docs/architecture/backend.md).
+
+## Quick Start
+
+### Prerequisites
+
+- Go 1.24+
+- Task (task runner)
+- golangci-lint (for linting)
+
+Install golangci-lint:
+
+```bash
+# macOS
+brew install golangci-lint
+
+# Windows
+go install github.com/golangci/golangci-lint/cmd/golangci-lint@latest
+
+# Linux
+curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(go env GOPATH)/bin
+```
+
+### Installation
+
+```bash
+task be:tidy
+task be:build
+```
+
+### Running the Server
+
+```bash
+task be:run
+```
+
+The server starts on port 4000 with:
+- WebSocket endpoint: `ws://localhost:4000/ws`
+- HTTP API: `http://localhost:4000/api/`
+
+### Running Tests
+
+```bash
+# Run all unit tests
+task be:test
+
+# Run tests in short mode (faster)
+task be:test:short
+```
+
+### Running Checks
+
+```bash
+# Run all checks (build, vet, test)
+task be:check
+
+# Individual checks
+task be:lint        # Linting with golangci-lint
+task be:format      # Format code with go fmt
+task be:vet         # Static analysis with go vet
+```
+
+## API Endpoints
+
+| Endpoint | Method | Purpose |
+|----------|--------|---------|
+| `/ws` | GET | WebSocket upgrade |
+| `/api/maps` | GET | List available maps |
+| `/api/createGame` | POST | Create new game room |
+| `/api/joinGame` | POST | Join existing room |
+| `/api/rooms/{id}/state` | GET | Get current game state |
+| `/api/rooms/{id}/reset` | POST | Reset game (training) |
+| `/api/rooms/{id}/stats` | GET | Get kill/death stats |
+| `/api/rooms/{id}/players/{pid}/action` | POST | Submit bot actions |
+| `/api/training/sessions` | GET | List training sessions |