Add an in-process post event bus (#13)

Handlers emit PostCreated and PostUpdated after a successful write. Store writes do not publish, and hidden roots are skipped.

Reviewed-on: #13
Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/events/event.go

@@ -0,0 +1,47 @@
+package events
+
+import (
+	"net/url"
+	"strings"
+)
+
+// Image is a public photo already attached to a site post.
+type Image struct {
+	URL         string
+	Description string
+}
+
+// PostEvent is a Discord-free snapshot of a site post after a successful write.
+type PostEvent struct {
+	PostID     string
+	RootID     string
+	ParentID   string
+	Title      string
+	Body       string
+	City       string
+	AuthorID   string
+	AuthorName string
+	AuthorRole string
+	Images     []Image
+	Permalink  string
+}
+
+// PostCreated is emitted after a successful site create.
+type PostCreated struct {
+	PostEvent
+}
+
+// PostUpdated is emitted after a successful site edit.
+type PostUpdated struct {
+	PostEvent
+}
+
+// Permalink builds /questions/{root}#post-{id}, prefixed by baseURL when set.
+func Permalink(baseURL, rootID, postID string) string {
+	path := "/questions/" + url.PathEscape(rootID) + "#post-" + url.PathEscape(postID)
+	base := strings.TrimRight(strings.TrimSpace(baseURL), "/")
+	if base == "" {
+		return path
+	}
+	return base + path
+}