fix: embedding URL path missing /v1 prefix

The embedding endpoint path was  but OpenAI-compatible APIs
serve embeddings at . When EMBEDDING_BASE_URL is set to a
bare host (e.g. https://llm.aviron.ai), the resulting URL would be
/eembeddings instead of /v1/embeddings, returning 404.

Changed defaultBaseURL to not include /v1 and always use /v1/embeddings
as the endpoint path.

flake.nix

@@ -77,8 +77,8 @@
 
             embeddingBaseUrl = lib.mkOption {
               type = lib.types.str;
-              default = "https://api.openai.com/v1";
-              description = "Base URL for the embeddings API.";
+              default = "https://api.openai.com";
+              description = "Base URL for the embeddings API. Do not include /v1 suffix; it is appended automatically.";
             };
 
             embeddingModel = lib.mkOption {