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/db/engine.py

@@ -0,0 +1,54 @@
+import aiosqlite
+
+from bot.config import settings
+
+_db: aiosqlite.Connection | None = None
+
+SCHEMA = """\
+CREATE TABLE IF NOT EXISTS users (
+    telegram_id  INTEGER PRIMARY KEY,
+    username     TEXT,
+    first_name   TEXT,
+    lang         TEXT NOT NULL DEFAULT 'en',
+    free_used    INTEGER NOT NULL DEFAULT 0,
+    created_at   TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE TABLE IF NOT EXISTS analyses (
+    id           INTEGER PRIMARY KEY AUTOINCREMENT,
+    telegram_id  INTEGER NOT NULL REFERENCES users(telegram_id),
+    channel      TEXT NOT NULL,
+    depth        TEXT NOT NULL,
+    focus_areas  TEXT NOT NULL,
+    model_id     TEXT NOT NULL,
+    stars_paid   INTEGER NOT NULL DEFAULT 0,
+    report_path  TEXT,
+    created_at   TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE TABLE IF NOT EXISTS payments (
+    id                  INTEGER PRIMARY KEY AUTOINCREMENT,
+    telegram_id         INTEGER NOT NULL REFERENCES users(telegram_id),
+    telegram_payment_id TEXT NOT NULL UNIQUE,
+    stars_amount        INTEGER NOT NULL,
+    analysis_id         INTEGER REFERENCES analyses(id),
+    created_at          TEXT NOT NULL DEFAULT (datetime('now'))
+);
+"""
+
+
+async def get_db() -> aiosqlite.Connection:
+    global _db
+    if _db is None:
+        _db = await aiosqlite.connect(settings.db_path)
+        _db.row_factory = aiosqlite.Row
+        await _db.executescript(SCHEMA)
+        await _db.commit()
+    return _db
+
+
+async def close_db() -> None:
+    global _db
+    if _db is not None:
+        await _db.close()
+        _db = None