Feat: Add free.direct-img.link and ?i= result picker (#1)

- free.direct-img.link serves only unrestricted (public domain/CC0) images
  from Openverse (license=cc0,pdm) and Wikimedia Commons
  (haslicense:unrestricted), with optional ?src=openverse|wikimedia
- ?i=1..20 picks the i-th working image on both sites; broken links are
  skipped so every index is a different image
- Openverse OAuth token is cached in KV and refreshed automatically
- Skip Flickr "photo unavailable" placeholders (served with HTTP 200)
- Free/indexed cache keys use uppercase prefixes; plain main-site keys
  are unchanged so existing cache entries stay valid
- Update README, homepage, privacy policy and terms

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

functions/_utils/free.js

@@ -0,0 +1,21 @@
+import { openverseImageSearch } from "./openverse.js";
+import { wikimediaImageSearch } from "./wikimedia.js";
+
+// Wikimedia's User-Agent policy requires contact info; also used when downloading free images
+export const FREE_UA = "direct-img.link/1.0 (https://direct-img.link; contact@direct-img.link)";
+
+// Tried in this order when no ?src= is given
+const SOURCES = { openverse: openverseImageSearch, wikimedia: wikimediaImageSearch };
+
+// ?src=openverse|wikimedia, or "auto" (every source in order) when omitted. Returns null if invalid.
+export function parseFreeSource(searchParams) {
+  const src = searchParams.get("src")?.toLowerCase() || "auto";
+  return src === "auto" || Object.hasOwn(SOURCES, src) ? src : null;
+}
+
+// Yields each source's image URLs in turn; a source is only searched once the previous one runs out
+export async function* freeImageUrls(query, src, env) {
+  for (const name of src === "auto" ? Object.keys(SOURCES) : [src]) {
+    yield* (await SOURCES[name](query, env, FREE_UA)) || [];
+  }
+}