fix(exam): allow any institute teacher to fetch template source PDF

Removed the teacher_id ownership check from _require_source_visibility_or_404.
RLS already ensures a teacher can only see templates in their institute;
the ownership gate was blocking shared templates (e.g. board-uploaded AQA papers)
for any teacher who didn't personally create them.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

routers/exam/templates.py

@@ -69,9 +69,7 @@ def _require_owner(ctx: ExamContext, template: Dict[str, Any]) -> None:
 
 
 def _require_source_visibility_or_404(ctx: ExamContext, template: Dict[str, Any]) -> None:
-    """Template source reads must not leak existence across institutes or non-owners."""
-    if template.get("teacher_id") != ctx.user_id:
-        raise HTTPException(status_code=404, detail="Template not found")
+    """Institute boundary check — RLS already gates template visibility; this prevents cross-institute PDF leakage."""
     if template.get("institute_id") not in ctx.institute_ids:
         raise HTTPException(status_code=404, detail="Template not found")