feat: Request-ID middleware, expanding ring matchmaking, Redis Sentinel/Cluster

[Maryermarh]

Summary

This PR implements three backend improvements and confirms one already-fixed issue:

1. Request-ID Correlation Tracing Middleware (#1025)

• Added RequestIdMiddleware that reads incoming X-Request-ID header or generates a UUID v4
• Attaches request_id to tracing span context for all downstream logs
• Returns X-Request-ID header in all HTTP responses
• Unit tests verify header injection and response matching

2. Redis Expansion-Ring Matchmaking (#1012)

• Updated find_rated_match to use expanding search window: starts at +/-50 Elo, expands by +25 Elo every 5 seconds, caps at +/-300 Elo
• Atomic Lua script ensures concurrency-safe matching without race conditions
• Players find tight matches instantly during high traffic, window expands smoothly during low traffic

3. Redis Sentinel/Cluster Support (#1026)

• Updated create_redis_pool with support for redis://, redis+sentinel://, and redis+cluster:// URL schemes
• Added create_redis_pool_with_nodes for explicit multi-node configuration
• Added create_redis_pool_from_env that reads REDIS_NODES (comma-separated) and REDIS_CLUSTER env vars
• Added /health/redis endpoint reporting Redis connection status and ping latency

4. AuthProvider Already Present (#803)

• Confirmed AuthProvider is already wrapping children in frontend/app/layout.tsx at the root layout level

Closes #1025
Closes #1012
Closes #1026
Closes #803

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