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/drizzle/meta/_journal.json

@@ -218,6 +218,13 @@
       "when": 1772150400000,
       "tag": "0030_remove_task_approval",
       "breakpoints": true
+    },
+    {
+      "idx": 31,
+      "version": "6",
+      "when": 1772236800000,
+      "tag": "0031_add_phase_merge_base",
+      "breakpoints": true
     }
   ]
 }