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