Replace type casting with type predicate in relay

Replaces unsafe type casting with a type predicate function following
Fedify's established patterns (similar to isActor()).

Changes:
- Add isRelayFollower() type predicate in types.ts
- Replace "value as RelayFollower" cast with isRelayFollower(value) in builder.ts
- Remove unused RelayFollower type import

Benefits:
- Compile-time type narrowing after guard check
- Runtime validation of KV store data
- Consistent with Fedify's type-safe design philosophy
- No unsafe type assertions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: sij411

packages/relay/src/builder.ts

@@ -9,8 +9,8 @@ import {
 import { Application, isActor, Object } from "@fedify/fedify/vocab";
 import type { Actor } from "@fedify/fedify/vocab";
 import {
+  isRelayFollower,
   RELAY_SERVER_ACTOR,
-  type RelayFollower,
   type RelayOptions,
 } from "./types.ts";
 
@@ -79,9 +79,8 @@ async function getFollowerActors(
   const actors: Actor[] = [];
 
   for await (const { value } of ctx.data.kv.list(["follower"])) {
-    const follower = value as RelayFollower;
-    if (!follower) continue;
-    const actor = await Object.fromJsonLd(follower.actor);
+    if (!isRelayFollower(value)) continue;
+    const actor = await Object.fromJsonLd(value.actor);
     if (!isActor(actor)) continue;
     actors.push(actor);
   }

packages/relay/src/types.ts

@@ -37,3 +37,21 @@ export interface RelayFollower {
   readonly actor: unknown;
   readonly state: "pending" | "accepted";
 }
+
+/**
+ * Type predicate to check if a value is a valid RelayFollower.
+ * Provides both runtime validation and compile-time type narrowing.
+ *
+ * @param value The value to check
+ * @returns true if the value is a RelayFollower
+ */
+export function isRelayFollower(value: unknown): value is RelayFollower {
+  if (!value || typeof value !== "object") return false;
+  const obj = value as Record<string, unknown>;
+  return (
+    "actor" in obj &&
+    "state" in obj &&
+    typeof obj.state === "string" &&
+    (obj.state === "pending" || obj.state === "accepted")
+  );
+}