fix(chat): migrate legacy cache to Elio conversation

src/lib/chat/chat_cache_keys.ts

@@ -0,0 +1,29 @@
+import type { ChatMediaKind } from "@/data/schemas/chat";
+
+const CHARACTER_NAMESPACE = "::character:";
+const LEGACY_OWNER_KEY_PATTERN = /^(?:user|device):.+$/u;
+
+export function buildChatConversationKey(
+  ownerKey: string,
+  characterId: string,
+): string {
+  if (ownerKey.trim().length === 0 || characterId.trim().length === 0) {
+    throw new Error("Chat owner and character identities must not be empty.");
+  }
+  return `${ownerKey}${CHARACTER_NAMESPACE}${encodeURIComponent(characterId)}`;
+}
+
+export function buildChatMediaCacheKey(input: {
+  ownerKey: string;
+  messageId: string;
+  kind: ChatMediaKind;
+}): string {
+  return `${input.ownerKey}:${input.messageId}:${input.kind}`;
+}
+
+export function isLegacyChatCacheOwnerKey(ownerKey: string): boolean {
+  return (
+    !ownerKey.includes(CHARACTER_NAMESPACE) &&
+    LEGACY_OWNER_KEY_PATTERN.test(ownerKey)
+  );
+}