feat(backend): implement WebSocket session management

[Fang0067]

Overview

Implements WebSocket session management in arenax-backend featuring heartbeat management, timeout enforcement, session state preservation across reconnects, connection concurrency limiting per user, and reconnection metrics tracking.

Related Issue

Closes #870

Changes

WebSocket Realtime Session Management

• [MODIFY] backend/src/realtime/session_registry.rs
  • Added HEARTBEAT_INTERVAL (30s), CLIENT_TIMEOUT (90s / 3 missed heartbeats), STATE_PRESERVATION_TTL (60s), and MAX_CONNECTIONS_PER_USER (5).
  • Added session state preservation (PreservedSession) during disconnection to allow client subscription restoration upon reconnection within 60s.
  • Added reconnect handler to restore subscriptions, enforce user validation, and maintain reconnect metrics (reconnect_count, reconnect_failures, reconnect_expired).
  • Enforced max 5 connections per user in register and register_with_limit.
  • Added unit test suite for registry lifecycle, timeouts, limits, and reconnect metrics.
• [MODIFY] backend/src/realtime/user_ws.rs
  • Configured heartbeat interval (30s) and timeout (90s).
  • Enforced connection limit upon actor startup and handled reconnection flow to restore subscribed channels.
• [MODIFY] backend/tests/session_registry_test.rs
  • Added comprehensive integration test suite covering connection limits, heartbeats, disconnect state preservation, reconnection within and beyond TTL, and metric counters.

Verification Results

All session management components and test suites implemented and verified:
- Connection limits (max 5 per user)
- Heartbeat tracking and 3-beat (90s) timeout
- State preservation across 60s window
- Reconnect metric telemetry (count, failure, expired)

Acceptance Criteria	Status
Heartbeat every 30s (HEARTBEAT_INTERVAL = Duration::from_secs(30))	✅ Implemented in session_registry.rs & user_ws.rs
3 missed beats closes connection (CLIENT_TIMEOUT = Duration::from_secs(90))	✅ Implemented in session_registry.rs & user_ws.rs
State preserved 60s for reconnect (STATE_PRESERVATION_TTL = Duration::from_secs(60))	✅ Implemented in session_registry.rs
Max 5 connections per user (MAX_CONNECTIONS_PER_USER = 5)	✅ Implemented in session_registry.rs & user_ws.rs
Metrics on reconnects (reconnect_count, reconnect_failures, reconnect_expired)	✅ Implemented in session_registry.rs

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