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/euro.go

@@ -189,6 +189,10 @@ func (p *EuroPlugin) credit(userID id.UserID, amount float64, reason string) {
 // Debit subtracts euros atomically. Returns false if this would exceed debt limit.
 // Uses a conditional UPDATE to prevent race conditions (check-and-act in one statement).
 func (p *EuroPlugin) Debit(userID id.UserID, amount float64, reason string) bool {
+	if amount <= 0 {
+		slog.Warn("euro: Debit called with non-positive amount", "user", userID, "amount", amount)
+		return false
+	}
 	p.ensureBalance(userID)
 	d := db.Get()
 
@@ -216,6 +220,10 @@ func (p *EuroPlugin) Debit(userID id.UserID, amount float64, reason string) bool
 
 // Credit adds euros (exported for other plugins).
 func (p *EuroPlugin) Credit(userID id.UserID, amount float64, reason string) {
+	if amount <= 0 {
+		slog.Warn("euro: Credit called with non-positive amount", "user", userID, "amount", amount)
+		return
+	}
 	p.ensureBalance(userID)
 	p.credit(userID, amount, reason)
 }