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/plugins/translate/dictionary.py

@@ -658,6 +658,10 @@ class DictionaryManager:
             if not all_entries:
                 return []
 
+            # PERFORMANCE NOTE: O(rows × entries) regex matching per batch.
+            # Acceptable for current scale (50-100 rows × <1000 entries).
+            # For larger batches (>500 rows or >5000 entries), consider
+            # replacing with an Aho-Corasick automaton (e.g., pyahocorasick).
             matched: list[dict[str, Any]] = []
             seen_source_match: set = set()
 

backend/src/plugins/translate/service.py

@@ -825,13 +825,19 @@ class InlineCorrectionService:
 class BulkFindReplaceService:
     """Handle bulk find-and-replace on TranslationLanguage entries."""
 
+    MAX_PATTERN_LENGTH = 500
+
     @staticmethod
     def _compile_pattern(pattern: str, is_regex: bool) -> re.Pattern:
         """Compile a search pattern (regex or plain text)."""
-        import re
-        if is_regex:
-            return re.compile(pattern)
-        return re.compile(re.escape(pattern))
+        if len(pattern) > BulkFindReplaceService.MAX_PATTERN_LENGTH:
+            raise ValueError(
+                f"Pattern too long (max {BulkFindReplaceService.MAX_PATTERN_LENGTH} characters)"
+            )
+        try:
+            return re.compile(pattern) if is_regex else re.compile(re.escape(pattern))
+        except re.error as e:
+            raise ValueError(f"Invalid regex pattern: {e}")
 
     @staticmethod
     def _find_matching_entries(