eat: Shift to Input-Based Queue Tracking (Overlay Pattern)

[6reenhorn]

Summary

This PR refactors the queue join process to align with the project's goal of being a "Remote Tracking Overlay." Instead of the backend auto-assigning numbers, users now provide the physical ticket number they received from the establishment.

Key Changes

• Input-Based Join: Users now POST their existing queue_number to join.

Smart Validation

• Prevents users from joining a queue that has already been served.
• Prevents multiple active sessions from tracking the same ticket number at the same institution.

Reliability

• Added QueueJoinViewTests covering success and failure (validation) scenarios.

Testing Performed

• pytest queue_tracker/tests.py (19 passing)
• Manual verification of "already served" and "duplicate tracking" error messages.