refactor: add githubFetch helper to centralize GitHub API fetch logic (#959)

src/lib/github-fetch.ts

@@ -0,0 +1,87 @@
+/**
+ * Typed GitHub API fetch helper.
+ * Centralises Authorization headers, Accept header, ok-check,
+ * and 403/429 rate-limit error handling so metric routes don't
+ * repeat the same ~10-line pattern.
+ */
+
+import { GITHUB_API } from "@/lib/github";
+
+export { GITHUB_API };
+
+export class GitHubRateLimitError extends Error {
+  constructor(public resetAt: Date | null) {
+    super("GitHub API rate limit exceeded");
+    this.name = "GitHubRateLimitError";
+  }
+}
+
+export class GitHubApiError extends Error {
+  constructor(public status: number) {
+    super(`GitHub API error: ${status}`);
+    this.name = "GitHubApiError";
+  }
+}
+
+/**
+ * Fetch a GitHub API endpoint with standard headers.
+ * Throws GitHubRateLimitError on 403/429, GitHubApiError on other non-ok responses.
+ */
+export async function githubFetch<T>(
+  url: string,
+  token: string,
+  options: RequestInit = {}
+): Promise<T> {
+  const res = await fetch(url, {
+    ...options,
+    headers: {
+      Authorization: `Bearer ${token}`,
+      Accept: "application/vnd.github+json",
+      ...((options.headers as Record<string, string>) ?? {}),
+    },
+    cache: (options.cache as RequestCache) ?? "no-store",
+  });
+
+  if (res.status === 403 || res.status === 429) {
+    const resetHeader = res.headers.get("X-RateLimit-Reset");
+    const resetAt = resetHeader ? new Date(Number(resetHeader) * 1000) : null;
+    throw new GitHubRateLimitError(resetAt);
+  }
+
+  if (!res.ok) {
+    throw new GitHubApiError(res.status);
+  }
+
+  return res.json() as Promise<T>;
+}
+
+/**
+ * POST to GitHub GraphQL API.
+ */
+export async function githubGraphQL<T>(
+  query: string,
+  token: string
+): Promise<T> {
+  const res = await fetch("https://api.github.com/graphql", {
+    method: "POST",
+    headers: {
+      Authorization: `Bearer ${token}`,
+      "Content-Type": "application/json",
+    },
+    body: JSON.stringify({ query }),
+    cache: "no-store",
+  });
+
+  if (res.status === 403 || res.status === 429) {
+    const resetHeader = res.headers.get("X-RateLimit-Reset");
+    const resetAt = resetHeader ? new Date(Number(resetHeader) * 1000) : null;
+    throw new GitHubRateLimitError(resetAt);
+  }
+
+  if (!res.ok) {
+    throw new GitHubApiError(res.status);
+  }
+
+  const json = await res.json();
+  return json.data as T;
+}