Full codebase audit: 21 security/robustness fixes, 328 tests across 4 packages

Security & economy:
- Credit()/Debit() reject non-positive amounts (closes infinite-money exploit)
- Lottery ticket purchase uses in-transaction count check (closes TOCTOU race)
- Arena bail channel ownership prevents double-close panic
- Entry.ID validated before exec.Command in fetch-esteemed
- Internal errors no longer leaked to users (forex, esteemed)

Robustness:
- safeGo() panic recovery added to 17 goroutine launch sites across 14 plugins
- db.Close() added and called on shutdown
- Miniflux mutex pattern fixed (snapshot-under-lock)
- Silently ignored DB/JSON errors now logged (lottery_db)

Performance:
- Added indexes: idx_arena_runs_user(user_id, status), idx_euro_bal_user(user_id)

UX:
- Empty !buy args shows usage instead of "No item matching ''"
- "Failed to load character" now suggests !adventure

Test coverage:
- internal/util/parser_test.go (19 tests: XP, levels, progress bar, commands, parsing)
- internal/crypto/crypto_test.go (12 tests: encrypt/decrypt, HMAC, ParseKey)
- internal/plugin/helpers_test.go (30 tests: formatNumber, calc, lottery, chat perks)
- internal/plugin/audit_fixes_test.go (25 tests: safeGo, bail channels, error leaks, overflow)
- internal/db/db_test.go (4 tests: Init, Close, schema indexes)

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

internal/plugin/adventure_shop.go

@@ -101,7 +101,7 @@ func (p *AdventurePlugin) shopScheduleBrowseNudge(userID id.UserID) {
 	p.shopSessions.Store(string(userID)+":nudge_gen", gen)
 
 	capturedGen := gen
-	go func() {
+	safeGo("shop-nudge", func() {
 		time.Sleep(2 * time.Minute)
 		// Check if this goroutine is still the latest.
 		if val, ok := p.shopSessions.Load(string(userID) + ":nudge_gen"); !ok || val.(int64) != capturedGen {
@@ -117,7 +117,7 @@ func (p *AdventurePlugin) shopScheduleBrowseNudge(userID id.UserID) {
 		}
 		flavor, _ := advPickFlavor(luigiBrowseTimeout, userID, "luigi_browse")
 		p.SendDM(userID, fmt.Sprintf("*%s*", flavor))
-	}()
+	})
 }
 
 // ── Display: Luigi Greeting + Category Menu ─────────────────────────────────