- When only one Ollama server is configured, LLMConnectionError no longer
  adds it to the dead-server blacklist. This fixes the bug where a
  transient failure permanently blocked all requests until server restart.
- LLMModelNotFoundError on a single server is also not blacklisted.
- _discover_backends now falls back to settings.ollama_host when the
  ollama_backends_file is empty, missing, or returns no valid servers.
- Added unit tests covering single-server no-blacklist, multi-server
  blacklist, model-not-found no-blacklist, and empty-file fallback.

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