feat(presence): add distributed presence service for the social layer

[AlpNuhoglu]

Introduce a standalone Presence Service as the foundation for friends, parties, invites, reconnect and notifications. Presence is modeled as a distributed system, not an online/offline boolean.

Design:

• Redis is the source of truth: presence:{id} JSON (state, last_seen, connection_count) with a TTL. WS replicas heartbeat every 15s to refresh a 45s TTL, so presence self-heals after crashes/partitions with no explicit disconnect — a missing key is OFFLINE.
• All mutations are Lua scripts (atomic read-modify-write), so any WS replica can update any player with no sticky sessions and no leader election — horizontally scalable.
• Multi-device: connection_count keeps a player ONLINE until the last connection closes.
• States OFFLINE/ONLINE/IN_QUEUE/IN_MATCH/AWAY with a deliberately permissive transition rule (only OFFLINE->IN_QUEUE/IN_MATCH rejected) so future reconnect/rematch/party flows need no state-machine changes.
• Publishes PresenceOnline/Offline/StateChanged to NATS (events.presence, 15m retention) via the shared events.Publisher with trace propagation; consumers are optional. Reads are side-effect free.
• Bulk friend lookup (POST /presence/friends) via a single Redis pipeline: O(N) commands, one round trip, no N+1.

WS gateway feeds presence over HTTP through an injected, optional notifier (nil = no-op) — the only coupling, no WS internals touched. Adds metrics, tracing spans, k8s manifest (2 replicas), Dockerfile, compose, and docs/presence.md. matchmaking is unchanged.