fix(translate): M1-M3 medium + L1-L2 low bugs from QA review

M1: ReDoS guard — 500 char max + try/except on re.compile
M2: Performance note — O(n×m) regex documented for future optimization
M3: Verified test key names correct (metrics.py transforms cumulative_* → short)
L1: Removed @staticmethod no-op from module-level function
L2: Added aria-label to emoji indicators in CorrectionCell

backend/src/api/routes/translate/_helpers.py

@@ -37,7 +37,6 @@ def _run_to_response(run: Any) -> dict:
 
 # #region _dict_to_response [C:2] [TYPE Function]
 # @BRIEF Convert a TerminologyDictionary ORM model to a response dict with computed entry_count.
-@staticmethod
 def _dict_to_response(d: Any, entry_count: int = 0) -> dict:
     return {
         "id": d.id,