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/i18n/__init__.py

@@ -0,0 +1,30 @@
+from enum import Enum
+
+from bot.i18n.strings import STRINGS
+
+
+class Lang(Enum):
+    EN = "en"
+    RU = "ru"
+
+
+def t(key: str, lang: Lang | str = Lang.EN, **kwargs: object) -> str:
+    if isinstance(lang, Lang):
+        lang_str = lang.value
+    elif lang in ("en", "ru"):
+        lang_str = lang
+    else:
+        lang_str = "en"
+    entry = STRINGS.get(key)
+    if not entry:
+        return key
+    text = entry.get(lang_str, entry.get("en", key))
+    if kwargs:
+        text = text.format(**kwargs)
+    return text
+
+
+def detect_lang(language_code: str | None) -> Lang:
+    if language_code and language_code.startswith("ru"):
+        return Lang.RU
+    return Lang.EN