feat(export): download project photos as a grouped zip

Add POST /api/export {projectIds}: one folder per project (named after it),
photos named <lastname-firstname>.<ext>, with a generated initials SVG for
Members without a Slack photo. Dedupe image fetches, bounded concurrency, fflate
zip. Factor data.ts so routes + export share one fixtures/live gating path.
Frontend: Export-photos button downloads the zip.

backend/src/data.ts

@@ -0,0 +1,23 @@
+import { hasNaptaCredentials } from "./config.ts"
+import { fixtureMembers, fixtureProjects } from "./fixtures.ts"
+import { fetchNaptaProjectMembers, fetchNaptaProjects } from "./napta.ts"
+import { type EnrichResult, enrichWithSlackAvatars } from "./slack.ts"
+import type { DataSource, Project } from "./types.ts"
+
+// Single source of truth for "fixtures vs live", shared by the API routes and
+// the Export so the gating logic isn't duplicated.
+
+export async function getProjects(): Promise<{ source: DataSource; projects: Project[] }> {
+  if (!hasNaptaCredentials) return { source: "fixture", projects: fixtureProjects }
+  return { source: "napta", projects: await fetchNaptaProjects() }
+}
+
+export async function getProjectMembers(
+  projectId: string,
+): Promise<{ source: DataSource } & EnrichResult> {
+  if (!hasNaptaCredentials) {
+    return { source: "fixture", members: fixtureMembers(projectId), slack: "unconfigured" }
+  }
+  const enriched = await enrichWithSlackAvatars(await fetchNaptaProjectMembers(projectId))
+  return { source: "napta", ...enriched }
+}