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/services/report_saver.py

@@ -0,0 +1,18 @@
+import os
+from datetime import datetime, timezone
+
+from bot.config import settings
+
+
+async def save_report(
+    telegram_id: int,
+    channel: str,
+    report_md: str,
+) -> str:
+    os.makedirs(settings.reports_dir, exist_ok=True)
+    ts = datetime.now(timezone.utc).strftime("%Y%m%d_%H%M%S")
+    filename = f"{telegram_id}_{channel}_{ts}.md"
+    path = os.path.join(settings.reports_dir, filename)
+    with open(path, "w", encoding="utf-8") as f:
+        f.write(report_md)
+    return path