feat(llm): OpenAI-compatible chatJSON helper with injectable fetch

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/lib/llm.ts

@@ -0,0 +1,38 @@
+import type { Db } from "../db";
+import { DEFAULTS, getSetting } from "../routes/settings";
+
+export type LlmDeps = { db: Db; key: Buffer; fetchImpl?: typeof fetch };
+
+export async function chatJSON(deps: LlmDeps, opts: { system: string; user: string }): Promise<unknown> {
+  const f = deps.fetchImpl ?? fetch;
+  const base = (await getSetting(deps.db, "llm_base_url", deps.key)) ?? DEFAULTS.llm_base_url;
+  const model = (await getSetting(deps.db, "llm_model", deps.key)) ?? DEFAULTS.llm_model;
+  const apiKey = await getSetting(deps.db, "llm_key", deps.key);
+
+  const headers: Record<string, string> = { "content-type": "application/json" };
+  if (apiKey) headers.authorization = `Bearer ${apiKey}`;
+
+  const res = await f(`${base.replace(/\/$/, "")}/chat/completions`, {
+    method: "POST",
+    headers,
+    body: JSON.stringify({
+      model,
+      response_format: { type: "json_object" },
+      messages: [
+        { role: "system", content: opts.system },
+        { role: "user", content: opts.user },
+      ],
+    }),
+  });
+  if (!res.ok) throw new Error(`llm_error: provider returned ${res.status}`);
+
+  const data = (await res.json()) as { choices?: { message?: { content?: string } }[] };
+  const content = data.choices?.[0]?.message?.content;
+  if (!content) throw new Error("llm_error: empty completion");
+  const stripped = content.replace(/^\s*```(?:json)?\s*/i, "").replace(/\s*```\s*$/, "");
+  try {
+    return JSON.parse(stripped);
+  } catch {
+    throw new Error("llm_error: completion was not valid JSON");
+  }
+}