refactor(translate): remove all deprecated columns from model + DB

Removed 6 legacy columns replaced by Phase 11 multi-language models:
- TranslationJob.target_language → target_languages (JSON)
- TranslationRecord.{llm_translation, user_edit, final_value} → TranslationLanguage
- TerminologyDictionary.{source_language, target_language} → DictionaryEntry per-entry

Affected files: model, schemas, 5 plugins, 4 test files
Alembic migration: aa1b2c3d4e5f (DROP COLUMN IF EXISTS)
Production DB: columns dropped via ALTER TABLE

Tests: 234 passed, 0 failed

backend/src/plugins/translate/orchestrator.py

@@ -106,7 +106,6 @@ class TranslationOrchestrator:
                 "translation_column": job.translation_column,
                 "target_column": job.target_column,
                 "context_columns": job.context_columns,
-                "target_language": job.target_language,
                 "provider_id": job.provider_id,
                 "batch_size": job.batch_size,
                 "upsert_strategy": job.upsert_strategy,
@@ -204,7 +203,7 @@ class TranslationOrchestrator:
             )
 
             # Initialize per-language stats
-            target_languages = job.target_languages or [job.target_language or job.target_dialect or "en"]
+            target_languages = job.target_languages or [job.target_dialect or "en"]
             if not isinstance(target_languages, list):
                 target_languages = [str(target_languages)]
             language_stats_map: dict[str, TranslationRunLanguageStats] = {}
@@ -1007,7 +1006,6 @@ class TranslationOrchestrator:
             "source_datasource_id": job.source_datasource_id,
             "translation_column": job.translation_column,
             "context_columns": job.context_columns,
-            "target_language": job.target_language,
             "target_languages": sorted(job.target_languages) if job.target_languages else [],
             "provider_id": job.provider_id,
             "batch_size": job.batch_size,