fix(chat): migrate legacy character cache identities

src/lib/chat/chat_cache_keys.ts

@@ -1,6 +1,9 @@
 import type { ChatMediaKind } from "@/data/schemas/chat";
+import { normalizeLegacyCharacterId } from "@/data/constants/character_id_aliases";
 
 const CHARACTER_NAMESPACE = "::character:";
+const LEGACY_OWNER_KEY_PATTERN = /^(?:user|device):.+$/u;
+const LEGACY_DEFAULT_CHARACTER_ID = "elio";
 
 export function buildChatConversationKey(
   ownerKey: string,
@@ -19,3 +22,37 @@ export function buildChatMediaCacheKey(input: {
 }): string {
   return `${input.ownerKey}:${input.messageId}:${input.kind}`;
 }
+
+/**
+ * Migrates both pre-character owner keys and keys containing the first
+ * frontend-only character IDs. This is intentionally idempotent so it can be
+ * used by database upgrades and compatibility reads.
+ */
+export function migrateLegacyChatConversationKey(ownerKey: string): string {
+  const namespaceIndex = ownerKey.lastIndexOf(CHARACTER_NAMESPACE);
+  if (namespaceIndex < 0) {
+    return LEGACY_OWNER_KEY_PATTERN.test(ownerKey)
+      ? buildChatConversationKey(ownerKey, LEGACY_DEFAULT_CHARACTER_ID)
+      : ownerKey;
+  }
+
+  const encodedCharacterId = ownerKey.slice(
+    namespaceIndex + CHARACTER_NAMESPACE.length,
+  );
+  if (encodedCharacterId.length === 0) return ownerKey;
+
+  let characterId: string;
+  try {
+    characterId = decodeURIComponent(encodedCharacterId);
+  } catch {
+    return ownerKey;
+  }
+
+  const normalizedCharacterId = normalizeLegacyCharacterId(characterId);
+  if (normalizedCharacterId === characterId) return ownerKey;
+
+  return buildChatConversationKey(
+    ownerKey.slice(0, namespaceIndex),
+    normalizedCharacterId,
+  );
+}