feat: multiple translations with context, improved task examples

- Add WordTranslation model for storing multiple translations per word
- AI generates translations with example sentences and their translations
- Show example usage after answering tasks (learning + interface language)
- Save translations to word_translations table when adding words from tasks
- Improve word exclusion in new_words mode (stronger prompt + client filtering)
- Add migration for word_translations table

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

locales/en.json

@@ -58,6 +58,7 @@
     "prompt": "Send the word you want to add:\nFor example: <code>/add elephant</code>\n\nOr just send the word without a command!",
     "searching": "⏳ Looking up translation and examples...",
     "examples_header": "<b>Examples:</b>",
+    "translations_header": "<b>Translations:</b>",
     "translation_label": "Translation",
     "category_label": "Category",
     "level_label": "Level",
@@ -132,6 +133,7 @@
     "add_word_btn": "➕ Add word",
     "word_added": "✅ Word '{word}' added to vocabulary!",
     "word_already_exists": "Word '{word}' is already in vocabulary",
+    "example_label": "Example",
     "cancelled": "Cancelled. You can return to tasks with /task.",
     "finish_title": "{emoji} <b>Task finished!</b>",
     "correct_of": "Correct answers: <b>{correct}</b> of {total}",