feat(translate): add cache_hits counter — backend + frontend

Backend:
- TranslationRun model: add cache_hits Integer column (default 0)
- TranslationRunResponse schema: add cache_hits field
- _helpers.py/_run_list_routes.py/orchestrator_aggregator.py: include
  cache_hits in all run API responses
- _batch_proc.py: count pre_rows served from translation cache per
  batch, return cache_hits in batch result
- executor.py: accumulate cache_hits across batches, persist to run
- Alembic migration: dabc9709 — add cache_hits column to translation_runs

Frontend:
- translationRun.svelte.ts: add cacheHits to store state, WS handler,
  polling handler
- TranslationRunProgress.svelte: 5-col stats grid with purple Cache card
- TranslationRunGlobalIndicator.svelte: 5-col stats with Cache
- TranslationRunResult.svelte: 5-col detail stats with Cache card
- History page: cache_hits shown in run list row + detail panel

Visual: cache hits shown in purple alongside green/yellow/red metrics
(total/success/failed/skipped). Visible during run + in history.

Tests: backend 69/69 translate tests ✅, frontend 698/698 tests ✅,
frontend build ✅

backend/src/plugins/translate/_batch_proc.py

@@ -73,6 +73,8 @@ class BatchProcessingService:
             llm_rows, pre_rows = self._classify(batch_rows, preview_edits_cache, tls)
 
             result["successful"] += self._persist_pre(pre_rows, bid, run_id, tls)
+            # Count cache hits: pre_rows that have _cached_lang_values (served from cache, not same-lang/approved)
+            result["cache_hits"] = sum(1 for r in pre_rows if r.get("_cached_lang_values"))
             if llm_rows:
                 llm_res = self._process_llm(job, run_id, llm_rows, dict_matches, bid, tls)
                 for k in ("successful", "failed", "skipped", "retries"):

backend/src/plugins/translate/executor.py

@@ -129,7 +129,7 @@ class TranslationExecutor:
                          target_languages: list[str],
                          language_stats_map: dict[str, TranslationRunLanguageStats] | None = None) -> TranslationRun:
         """Process all batches: execute, insert to target, check cancellation."""
-        successful_records = failed_records = skipped_records = 0
+        successful_records = failed_records = skipped_records = cache_hits = 0
         for batch_idx, batch_rows in enumerate(batches):
             batch_result = self._process_batch(
                 job=job, run_id=run.id, batch_index=batch_idx, batch_rows=batch_rows,
@@ -138,9 +138,11 @@ class TranslationExecutor:
             successful_records += batch_result.get("successful", 0)
             failed_records += batch_result.get("failed", 0)
             skipped_records += batch_result.get("skipped", 0)
+            cache_hits += batch_result.get("cache_hits", 0)
             run.successful_records = successful_records
             run.failed_records = failed_records
             run.skipped_records = skipped_records
+            run.cache_hits = cache_hits
             self.db.commit()
 
             batch_id = batch_result.get("batch_id")

backend/src/plugins/translate/orchestrator_aggregator.py

@@ -78,6 +78,7 @@ class TranslationResultAggregator:
                 "successful_records": run.successful_records or 0,
                 "failed_records": run.failed_records or 0,
                 "skipped_records": run.skipped_records or 0,
+                "cache_hits": run.cache_hits or 0,
                 "insert_status": run.insert_status,
                 "superset_execution_id": run.superset_execution_id,
                 "batch_count": batch_count,