fix: Show completed phase diffs in review tab

Completed phases showed "No phases pending review" because:
1. Frontend filtered only pending_review phases
2. Server rejected non-pending_review phases
3. After merge, three-dot diff returned empty (merge base moved)

Fix: store pre-merge merge base hash on phase, use it to reconstruct
diffs for completed phases. Frontend now shows both pending_review and
completed phases with read-only mode (Merged badge) for completed ones.

apps/server/git/simple-git-branch-manager.ts

@@ -141,6 +141,12 @@ export class SimpleGitBranchManager implements BranchManager {
     return git.diff([`${commitHash}~1`, commitHash]);
   }
 
+  async getMergeBase(repoPath: string, branch1: string, branch2: string): Promise<string> {
+    const git = simpleGit(repoPath);
+    const result = await git.raw(['merge-base', branch1, branch2]);
+    return result.trim();
+  }
+
   async pushBranch(repoPath: string, branch: string, remote = 'origin'): Promise<void> {
     const git = simpleGit(repoPath);
     await git.push(remote, branch);