fix(api): log test-connection failures with provider context

[funkypenguin]

Summary

The /api/test-usenet-connection endpoint silently swallows CouldNotConnectToUsenetException and CouldNotLoginToUsenetException and returns connected: false with no log line. Users hitting "Connection test failed" in the UI have no way to diagnose the actual cause — bad credentials, DNS failure, TLS handshake failure, server-side rejection, unreachable host, etc. — without attaching a debugger or packet-sniffing.

I hit this debugging a real provider misconfiguration this week: the UI showed the generic red banner and the pod logs were silent. The fix took five minutes once I knew the error message; finding the message took much longer.

Changes

backend/Api/Controllers/TestUsenetConnection/TestUsenetConnectionController.cs:

• Add Log.Warning to the CouldNotConnectToUsenetException catch — prefixed connect error.
• Add Log.Warning to the CouldNotLoginToUsenetException catch — prefixed login error.
• Add a catch for unexpected non-cancellation exceptions that logs with stack trace, then rethrows so BaseApiController's existing 500 handler still runs (preserves the frontend's "Failed to test connection" branch for non-NNTP failures).

Each log line includes host, port, SSL flag, user, and the inner exception message:

Test connection failed for news.example.com:563 (ssl=True, user=alice): login error: 481 Authentication failed
Test connection failed for news.example.com:563 (ssl=True, user=alice): connect error: The remote certificate is invalid according to the validation procedure

The wire-format response is unchanged — connected: false still goes back on the typed exceptions. This is purely additive observability.

Test plan

• dotnet build clean (0 errors, 77 pre-existing warnings)
• Manual: test connection with deliberately wrong password → log line with login error: 481 ...
• Manual: test connection with unreachable host → log line with connect error: ...
• Manual: test connection with valid credentials → no warning, connected: true returned

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