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/tests/test_translate_jobs.py

@@ -194,7 +194,7 @@ def test_create_job_valid(db_session):
     assert job.context_columns == ["description"]
     assert job.batch_size == 100
     assert job.upsert_strategy == "MERGE"
-    assert job.target_language == "ru"
+    assert job.target_languages == ["ru"]
     assert job.status == "DRAFT"
     assert job.created_by == "test_user"