fix(translate): prevent empty translations in _persist_pre + add diag logging

Bug 1 (secondary): _persist_pre() created TranslationLanguage with
empty final_value for non-source target languages when same_language
flag was set but no cache/approved value existed. This produced
is_original=0 rows with empty text in target table.

Fix: skip TranslationLanguage creation when fv is empty (continue
in the loop instead of falling through to else with empty string).

Bug 2 (diagnostic): No visibility into _classify() routing decisions.
Add logger.reason with pre/llm/total counts per batch.

Scenario trace: ru source + targets [ru,en] + no cache → llm_rows ✓
If same-language partial match hits cache for all targets → pre_rows ✓
If same-language partial match misses cache → llm_rows ✓
If same-language all-match (targets=[ru]) → short-circuit pre_rows ✓

Tests: 69/69 translate tests pass.

backend/src/plugins/translate/_batch_proc.py

@@ -160,6 +160,9 @@ class BatchProcessingService:
                             pre_rows.append(row)
                             continue
             llm_rows.append(row)
+        logger.reason("Batch classified",
+                      {"tls": tls, "pre": len(pre_rows), "llm": len(llm_rows),
+                       "total": len(pre_rows) + len(llm_rows)})
         return llm_rows, pre_rows
 
     def _persist_pre(self, pre_rows, bid, run_id, tls):
@@ -190,7 +193,8 @@ class BatchProcessingService:
                 elif cl and lc in cl:
                     fv = cl[lc]
                 else:
-                    fv = row.get("approved_translation", "")
+                    # No translation available — skip this language
+                    continue
                 self.db.add(TranslationLanguage(
                     id=str(uuid.uuid4()), record_id=rec.id, language_code=lc,
                     source_language_detected=detected_lang, translated_value=fv,