Address PR review comments: fix CI duplication, improve documentation, enhance error handling

[Copilot]

Addresses all outstanding PR review comments from the client-server architecture transition. Changes span CI configuration, documentation accuracy, and server-side code quality improvements.

CI & Build

• Workflows: Removed duplicate pull_request triggers causing double runs on PR creation
• Environment: Renamed .env.example → sample.env per convention

Documentation

• PR references: Updated ARCHITECTURE.md and README.md to reference PR Transition to Spring Boot Server & Pygame Client #266 instead of Implement client-server architecture with Spring Boot backend, REST API, pygame client, and Docker support #242
• Authentication flow: Corrected login UI instructions (button-based, not 'R' key)
• Removed obsolete files: demo_api.py, Dockerfile.client (non-functional)

Server Code Quality

• AuthController: Throws exception for missing Authorization header instead of null handling

  String token = parseJwt(request);
  if (token == null) {
      throw new RuntimeException("Missing or invalid Authorization header");
  }

• PlayerController: Clarified entity interaction semantics
  • Harvesting: extracts resources, entity persists with cooldown (Trees → Wood)
  • Gathering: picks up and removes entity (Apples, Berries)
• Rate limiting: Documented exception-based flow when limits exceeded
• Multi-player: Clarified isolated sessions per user

Technical Debt

• Added TODO for Lombok integration to reduce DTO boilerplate
• Added TODO for Grass entity implementation (currently tile-only)

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