Refactor ER diagram to add EventManager and update relations

Introduce the EventManager entity to improve role management, linking it between Users and Events. Adjust relationships among User, Event, and Guest, and refine EventTheme and User attributes. These changes enhance schema clarity and better represent user roles and permissions.

docs/eventspace-er-schema.mermaid

@@ -1,8 +1,10 @@
 erDiagram
-    User ||--o{ Event : "creates/manages"
+    User ||--o{ Event : "creates"
+    User ||--o{ EventManager : "is assigned as"
+    User ||--o{ Guest : "can be linked to"
     User {
         uuid id PK
-        string email
+        string email UK
         string password_hash
         string first_name
         string last_name
@@ -14,11 +16,28 @@ erDiagram
         json preferences
     }
 
+    EventManager }o--|| Event : "manages"
+    EventManager {
+        uuid id PK
+        uuid user_id FK
+        uuid event_id FK
+        string role
+        boolean can_edit
+        boolean can_invite
+        boolean can_manage_gifts
+        boolean can_send_updates
+        boolean can_view_analytics
+        datetime assigned_at
+        uuid assigned_by FK
+        datetime created_at
+        datetime updated_at
+    }
+
     Event ||--o{ Guest : "invites"
     Event ||--o{ GiftItem : "contains"
     Event ||--o{ EventMedia : "has"
     Event ||--o{ EventUpdate : "has"
-    Event ||--o{ EventTheme : "uses"
+    Event }o--|| EventTheme : "uses"
     Event {
         uuid id PK
         string title
@@ -53,6 +72,7 @@ erDiagram
     Guest {
         uuid id PK
         uuid event_id FK
+        uuid user_id FK "nullable"
         string name
         string email
         string phone
@@ -235,3 +255,4 @@ erDiagram
     Event ||--o{ RSVP : "receives"
     Event ||--o{ EventActivity : "logs"
     Event ||--o{ NotificationLog : "generates"
+    User ||--o{ EventManager : "has roles"