llm: add FastAPI shim, gateway LLM endpoints, tests, and docs

commit_output.txt

@@ -0,0 +1,9 @@
+On branch main
+Your branch is ahead of 'origin/main' by 1 commit.
+  (use "git push" to publish your local commits)
+
+Untracked files:
+  (use "git add <file>..." to include in what will be committed)
+	commit_output.txt
+
+nothing added to commit but untracked files present (use "git add" to track)