Add chat level perks: XP bonus, shop flavor, rare drop nudge, death pardon (Adventure 2.5 steps 3-6)

- XP bonus: +5% per 10 chat levels (cap +25% at 50+), applies to all
  adventure and arena XP
- Shop flavor: shopkeeper greeting changes at chat levels 10/30/50,
  replacing random flavor for recognized players
- Rare drop nudge: +0.5% per 10 chat levels (cap +2.5%) applied
  additively to treasure and masterwork drop rates
- Death pardon: chat level 20+ gets 33% chance to survive death once
  per 7 days (converts to bad-failure, quiet DM). Does not apply in
  arena. Fires before Sovereign Death's Reprieve check.
- New adventure_chat_perks.go with perk calculation helpers
- LastPardonUsed field + migration on adventure_characters

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

internal/db/db.go

@@ -85,6 +85,7 @@ func runMigrations(d *sql.DB) error {
 		`ALTER TABLE adventure_characters ADD COLUMN last_death_date TEXT NOT NULL DEFAULT ''`,
 		`ALTER TABLE adventure_characters ADD COLUMN combat_actions_used INTEGER NOT NULL DEFAULT 0`,
 		`ALTER TABLE adventure_characters ADD COLUMN harvest_actions_used INTEGER NOT NULL DEFAULT 0`,
+		`ALTER TABLE adventure_characters ADD COLUMN last_pardon_used DATETIME`,
 	}
 	for _, stmt := range columnMigrations {
 		if _, err := d.Exec(stmt); err != nil {