feat: add invite code validity controls

- Add invite code starts/expires fields across contracts, API, Spacetime bindings, and admin UI
- Enforce pending/expired invite code redemption behavior and expose admin status
- Add admin write-operation confirmation guard and documentation
- Add invite code contract/runtime tests

server-rs/crates/shared-contracts/src/runtime.rs

@@ -412,6 +412,10 @@ pub struct AdminUpsertProfileInviteCodeRequest {
     pub invite_code: String,
     #[serde(default)]
     pub metadata: Option<serde_json::Value>,
+    #[serde(default)]
+    pub starts_at: Option<String>,
+    #[serde(default)]
+    pub expires_at: Option<String>,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
@@ -447,6 +451,9 @@ pub struct ProfileInviteCodeAdminResponse {
     pub user_id: String,
     pub invite_code: String,
     pub metadata: serde_json::Value,
+    pub starts_at: Option<String>,
+    pub expires_at: Option<String>,
+    pub status: String,
     pub created_at: String,
     pub updated_at: String,
 }