refactor(domain): extract the KV key schema into feed-keys.ts

Move the feed:/icon:/websub: key builders out of FeedRepository into a
pure feed-keys module so the wire format lives in one place, shared by
the repositories to come. Strings are byte-identical; behaviour unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/domain/feed-keys.ts

@@ -0,0 +1,40 @@
+import { FEEDS_LIST_KEY, STATS_KEY } from "../config/constants";
+
+/**
+ * The KV key schema, in one pure place. Every repository builds its keys here so
+ * the wire format lives in a single module — never inline a `feed:`/`icon:`/
+ * `websub:` string elsewhere. Strings are byte-identical to the original schema;
+ * changing them would require migrating live KV data.
+ */
+
+const WEBSUB_PREFIX = "websub:subs:";
+
+export const feedKeys = {
+  config: (feedId: string): string => `feed:${feedId}:config`,
+  metadata: (feedId: string): string => `feed:${feedId}:metadata`,
+
+  /** Prefix covering every key owned by a feed (config, metadata, emails). */
+  feedPrefix: (feedId: string): string => `feed:${feedId}:`,
+
+  /** Mint a fresh, time-ordered email key. Call once and reuse the result. */
+  newEmail: (feedId: string): string => `feed:${feedId}:${Date.now()}`,
+
+  /** KV key for a domain's cached favicon (shared across feeds). */
+  icon: (domain: string): string => `icon:${domain}`,
+
+  websub: (feedId: string): string => `${WEBSUB_PREFIX}${feedId}`,
+
+  /** Prefix matching every per-feed WebSub subscription key. */
+  websubPrefix: (): string => WEBSUB_PREFIX,
+
+  /** True when `key` is an email entry (not the feed's config/metadata key). */
+  isEmail: (feedId: string, key: string): boolean => {
+    const suffix = key.slice(feedKeys.feedPrefix(feedId).length);
+    return suffix !== "config" && suffix !== "metadata";
+  },
+
+  /** Recover the feed id embedded in an email key (`feed:<id>:<ts>`). */
+  feedIdFromEmail: (key: string): string => key.split(":")[1],
+} as const;
+
+export { FEEDS_LIST_KEY, STATS_KEY };