docs: add conceptual documentation for presence and heartbeat services

[Samuel-21-eng]

The new documentation apps/backend/docs/concepts-presence-heartbeat.md provides a comprehensive overview of the
backend's presence and heartbeat mechanisms:

• Redis Key Architecture: Detailed breakdown of the Redis data structures (Hash, Set) used to track user presence,
  device sessions, and socket mappings, including their purpose and TTL logic.
• Heartbeat Contract: Defined the operational parameters, specifically the 90-second heartbeat timeout for
  connection liveness and the 30-second throttle for database updates to balance precision with performance.
• Multi-device & Aggregation Logic: Explained how the system aggregates multiple active devices to determine a
  user's overall "online" status and the conditions for broadcasting state transitions (user_offline).
• System Reconciliation: Documented the gateway boot reconciliation process to ensure real-time room memberships
  remain consistent after service restarts.

This document serves as the canonical reference for understanding how client connections are maintained, timed out,
and synchronized across the distributed backend architecture.

closes #447

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[codebestia]

Hello @Samuel-21-eng
Please remove the package-lock.json diffs from your PR