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/lang.py

@@ -0,0 +1,20 @@
+from aiogram import Router
+from aiogram.filters import Command
+from aiogram.types import Message
+
+from bot.db import user_repo
+from bot.i18n import Lang, t
+
+router = Router()
+
+
+@router.message(Command("lang"))
+async def cmd_lang(message: Message, lang: Lang = Lang.EN, **_: object) -> None:
+    args = (message.text or "").split(maxsplit=1)
+    if len(args) < 2 or args[1].strip().lower() not in ("en", "ru"):
+        await message.answer(t("lang_usage", lang), parse_mode="HTML")
+        return
+
+    new_lang = args[1].strip().lower()
+    await user_repo.update_lang(message.from_user.id, new_lang)
+    await message.answer(t("lang_set", new_lang), parse_mode="HTML")