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/api/routes/translate/_helpers.py

@@ -23,6 +23,7 @@ def _run_to_response(run: Any) -> dict:
         "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,
         "config_snapshot": run.config_snapshot,

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

@@ -104,6 +104,7 @@ async def list_runs(
                 "successful_records": r.successful_records or 0,
                 "failed_records": r.failed_records or 0,
                 "skipped_records": r.skipped_records or 0,
+                "cache_hits": r.cache_hits or 0,
                 "insert_status": r.insert_status,
                 "superset_execution_id": r.superset_execution_id,
                 "config_hash": r.config_hash,