feat: add nerve config and Character nerve methods

Add nerve system configuration to config/game.php with start_amount,
max, regen_rate, regen_interval, and midnight_refill settings. Add gym
configuration with energy_cost, daily_cap, and stat_gain. Add nerve
methods to Character model: hasNerve(), consumeNerve(), addNerve(), and
getNervePercentageAttribute(). Update levelUp() to increment max_nerve
by 2. Add nerve_cost to fillable arrays in Crime and CrimeAttempt models.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/CrimeAttempt.php

@@ -25,6 +25,7 @@ class CrimeAttempt extends Model
         'success',
         'reward',
         'experience_gained',
+        'nerve_cost',
         'attempted_at',
     ];