feat: proper visitor analytics system - New Visitor model (visitor_uid + fingerprint fallback, first_seen/last_seen/page_views, geo, device) - VisitorRepository with upsert + analytics queries - visitor_tracker.py: record_visitor() upserts on every session/init - session/init accepts visitor_uid in request body (HF-Spaces iframe-safe, no cookies) - 4 new analytics endpoints: /visitors /geo /devices /sessions/stats - geo.py: region extraction + X-Real-IP support - DB migration: visitor_uid column on chat_sessions - ARCHITECTURE.md updated

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