feat(models): add model health check support

[leoguillaume]

Overview

Adds per-model health status for routers the caller can access, using Redis provider metrics (normalized latency history + inflight) and a Little’s-law-style overload indicator (green / yellow / red).

• New GET /health/models endpoint (authenticated) returning [{ "id": "", "status": "green|yellow|red" }]
• GetHealthModelsUseCase: filters by user access, skips routers without providers, aggregates worst status across providers
• Metrics refactor: ModelMetricsLogger → ProviderMetricsLogger with RedisProviderMetricsLogger; provider-level metric logging in model clients
• ProviderRepository.get_all_providers(); Router.has_providers helper

Breaking changes:

• No breaking changes
• This PR contains breaking changes (explain below)

Check lists

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