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/plugin.go

@@ -678,6 +678,52 @@ func (b *Base) SendDM(userID id.UserID, text string) error {
 	return b.SendMessage(roomID, text)
 }
 
+// SendDMID sends a direct message and returns the event ID (for later editing).
+func (b *Base) SendDMID(userID id.UserID, text string) (id.EventID, error) {
+	roomID, err := b.GetDMRoom(userID)
+	if err != nil {
+		return "", err
+	}
+	return b.SendMessageID(roomID, text)
+}
+
+// EditMessage edits an existing message using Matrix m.replace relation.
+func (b *Base) EditMessage(roomID id.RoomID, eventID id.EventID, newText string) error {
+	newContent := textContent(newText)
+	content := &event.MessageEventContent{
+		MsgType: event.MsgText,
+		Body:    "* " + newContent.Body,
+		NewContent: &event.MessageEventContent{
+			MsgType:       newContent.MsgType,
+			Body:          newContent.Body,
+			Format:        newContent.Format,
+			FormattedBody: newContent.FormattedBody,
+		},
+		RelatesTo: &event.RelatesTo{
+			Type:    event.RelReplace,
+			EventID: eventID,
+		},
+	}
+	if newContent.Format == event.FormatHTML {
+		content.Format = event.FormatHTML
+		content.FormattedBody = "* " + newContent.FormattedBody
+	}
+	_, err := b.Client.SendMessageEvent(context.Background(), roomID, event.EventMessage, content)
+	if err != nil {
+		slog.Error("failed to edit message", "room", roomID, "event", eventID, "err", err)
+	}
+	return err
+}
+
+// EditDM edits a message in a user's DM room.
+func (b *Base) EditDM(userID id.UserID, eventID id.EventID, newText string) error {
+	roomID, err := b.GetDMRoom(userID)
+	if err != nil {
+		return err
+	}
+	return b.EditMessage(roomID, eventID, newText)
+}
+
 // UploadContent uploads data to the Matrix content repository and returns the MXC URI.
 func (b *Base) UploadContent(data []byte, contentType, filename string) (id.ContentURI, error) {
 	resp, err := b.Client.UploadBytesWithName(context.Background(), data, contentType, filename)
@@ -708,3 +754,16 @@ func (b *Base) SendImage(roomID id.RoomID, imgData []byte, filename, caption str
 	_, err = b.Client.SendMessageEvent(context.Background(), roomID, event.EventMessage, content)
 	return err
 }
+
+// safeGo runs fn in a goroutine with panic recovery.
+// Use instead of bare `go func()` to prevent daemon crashes.
+func safeGo(label string, fn func()) {
+	go func() {
+		defer func() {
+			if r := recover(); r != nil {
+				slog.Error("goroutine panic recovered", "label", label, "panic", r)
+			}
+		}()
+		fn()
+	}()
+}