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/features.py

@@ -0,0 +1,16 @@
+from aiogram import Router
+from aiogram.filters import Command
+from aiogram.types import Message
+
+from bot.i18n import Lang, t
+from bot.models import FocusArea
+
+router = Router()
+
+
+@router.message(Command("features"))
+async def cmd_features(message: Message, lang: Lang = Lang.EN, **_: object) -> None:
+    lines = [t("features_title", lang)]
+    for area in FocusArea:
+        lines.append(f"• {t(f'features_{area.value}', lang)}")
+    await message.answer("\n".join(lines), parse_mode="HTML")