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/db/schema.ts

@@ -55,6 +55,7 @@ export const phases = sqliteTable('phases', {
   status: text('status', { enum: ['pending', 'approved', 'in_progress', 'completed', 'blocked', 'pending_review'] })
     .notNull()
     .default('pending'),
+  mergeBase: text('merge_base'),
   createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),
   updatedAt: integer('updated_at', { mode: 'timestamp' }).notNull(),
 });