refactor(models): replace hardcoded strings with typed enums

- Add ReviewState enum (pending, approved, changes_requested, etc.)
- Add DEFAULT_REVISION constant for git HEAD references
- service/pr/reviews.rs: use ReviewState for review creation and
  submission state validation
- service/pr/core.rs: use MergeStrategyKind for merge strategy
  selection
- service/im/stages.rs: use StagePrivacyLevel for stage creation
- service/im/invitations.rs: use Role enum for invitation role
  defaults

models/common.rs

@@ -203,6 +203,12 @@ string_enum! {
     }
 }
 
+impl Default for Visibility {
+    fn default() -> Self {
+        Visibility::Unknown
+    }
+}
+
 string_enum! {
     pub enum Priority {
         None => "none",
@@ -792,3 +798,18 @@ string_enum! {
         Unknown => "unknown",
     }
 }
+
+string_enum! {
+    /// Pull request review states.
+    pub enum ReviewState {
+        Pending => "pending",
+        Approved => "approved",
+        ChangesRequested => "changes_requested",
+        Commented => "commented",
+        Dismissed => "dismissed",
+        Unknown => "unknown",
+    }
+}
+
+/// Default git revision reference.
+pub const DEFAULT_REVISION: &str = "HEAD";