fix(llm): isolate tenant endpoint credentials

[CryptoJones]

Summary

• Prevent tenant-controlled LLM base URLs from inheriting the operator instance API key.
• Mark resolved configs whose base URL came from tenant settings and route those requests through a public-IP connect guard.
• Reject tenant LLM base URLs that are invalid, localhost, private IP literals, or common internal hostnames while still allowing operator instance defaults to point at local models.
• Add regression tests for mixed override cases and endpoint policy behavior.
• Bump version to 1.7.8.

Closes #47

Verification

• dotnet test api/ApplyTrack.slnx --filter FullyQualifiedName~LlmConfigTests --no-restore
• dotnet build api/ApplyTrack.slnx --no-restore
• dotnet format api/ApplyTrack.slnx --verify-no-changes --no-restore --include api/ApplyTrack.Api/Llm/LlmConfig.cs api/ApplyTrack.Api/Llm/LlmEndpointPolicy.cs api/ApplyTrack.Api/Llm/OpenAiCompatibleLlmClient.cs api/ApplyTrack.Api.Tests/LlmConfigTests.cs api/ApplyTrack.Api/Endpoints/MaterialsEndpoints.cs api/ApplyTrack.Api.Tests/MaterialsEndpointTests.cs
• uv run --extra dev pytest -q
• git diff --check

Full .NET integration tests require the Podman/Testcontainers socket and should run in CI or via ./scripts/test-dotnet-podman.sh on a Podman-enabled host.