[BE-2] Implement Turn Management System

[sarkarshuvojit]

Parent Epic

#22

Description

Implement turn rotation logic so players take turns in order.

Requirements

• Track current player in RoomState (CurrentPlayer field exists)
• Initialize turn order when game starts (after all boards submitted)
• Rotate to next player after each valid move
• Skip players who are eliminated (all ships sunk)
• Broadcast turn changes to all players

Turn Order

• 3 players: A → B → C → A → ...
• If B is eliminated: A → C → A → C → ...

Events

• Outbound: NEXT_TURN (already defined)

Files to Modify

• internal/game/types.go - Turn tracking
• internal/api/handlers/room.go - Turn initialization
• internal/api/jobs/game_jobs.go - Turn rotation job

Unit Tests

• Turn initializes to first player
• Turn rotates correctly after move
• Eliminated players are skipped
• Turn broadcasts to all players

Acceptance Criteria

• Turn system implemented
• Players can only act on their turn
• Invalid turn attempts rejected with error
• Unit tests passing