internal: centralize size/length constants; better DoS protection

...also adds a bounded custom event registry for cardinality control but
I ran out of space in the commit message. Praise be to the long
descriptions...

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: Ic205f69804c7fb24c39fa84abdd9546b6a6a6964

internal/aggregate/custom_events.go

@@ -0,0 +1,56 @@
+package aggregate
+
+import "sync"
+
+// Maintains a bounded set of allowed custom event names
+type CustomEventRegistry struct {
+	mu            sync.RWMutex
+	events        map[string]struct{}
+	maxEvents     int
+	overflowCount int
+}
+
+// Creates a registry with a maximum number of unique event names
+func NewCustomEventRegistry(maxEvents int) *CustomEventRegistry {
+	return &CustomEventRegistry{
+		events:    make(map[string]struct{}, maxEvents),
+		maxEvents: maxEvents,
+	}
+}
+
+// Add attempts to add an event name to the registry
+// Returns the event name if accepted, "other" if rejected due to limit
+func (r *CustomEventRegistry) Add(eventName string) string {
+	// Fast path: check with read lock first
+	r.mu.RLock()
+	if _, exists := r.events[eventName]; exists {
+		r.mu.RUnlock()
+		return eventName
+	}
+	r.mu.RUnlock()
+
+	// Slow path: acquire write lock to add
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	// Double-check after acquiring write lock
+	if _, exists := r.events[eventName]; exists {
+		return eventName
+	}
+
+	// Check limit
+	if len(r.events) >= r.maxEvents {
+		r.overflowCount++
+		return "other"
+	}
+
+	r.events[eventName] = struct{}{}
+	return eventName
+}
+
+// Returns the number of events rejected due to limit
+func (r *CustomEventRegistry) OverflowCount() int {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	return r.overflowCount
+}