[S4.1-05] Add Game Attempt and User Stats Schema

[humanauction]

Labels: stage-4, backend, db, game
Depends on: S4.1-03

Acceptance Criteria:

• Game attempts table stores user, guess, correctness, and timestamp.
• User stats storage/aggregation model is defined and implemented.
• Migration is reversible and tested.
• Integrity constraints prevent invalid records.