Add multi-user features: i18n, payments, multi-model AI, focus areas

Replace single report type with composable analysis: depth levels
(basic/standard/full) + focus area multi-select (psychology, business,
marketing, content, audience, sentiment). Multi-step inline keyboard
flow guides users through selection.

- i18n: English + Russian, auto-detect from Telegram, /lang override
- AI providers: Anthropic + OpenRouter via AIClient abstraction
- Telegram Stars payments with per-depth pricing and free trial
- SQLite (aiosqlite) for users, analyses, payments tracking
- User middleware for auto-registration and language detection
- Report persistence: save .md locally, offer file download
- New commands: /features, /prices, /lang
- Composable prompt system: depth modifiers + focus area fragments

bot/handlers/payment.py

@@ -0,0 +1,9 @@
+from aiogram import Router
+from aiogram.types import PreCheckoutQuery
+
+router = Router()
+
+
+@router.pre_checkout_query()
+async def on_pre_checkout(query: PreCheckoutQuery, **_: object) -> None:
+    await query.answer(ok=True)