feat: add sender profile cache with LRU eviction

- SenderNameCache: TTL 10min, LRU max 2048 entries, getOrFetch pattern
- resolveSenderName: API call with cache integration
- Trigger uses cache for approval card resolutions and audit metadata
- Add unit tests (9) for cache lifecycle, LRU, expiry, and null handling

hub/src/feishu/senderCache.ts

@@ -0,0 +1,82 @@
+export interface SenderCacheOptions {
+  readonly ttlMs?: number;
+  readonly maxSize?: number;
+}
+
+interface SenderCacheEntry {
+  readonly name: string;
+  readonly expiresAt: number;
+}
+
+const DEFAULT_TTL_MS = 600_000;
+const DEFAULT_MAX_SIZE = 2_048;
+
+export class SenderNameCache {
+  private readonly ttlMs: number;
+  private readonly maxSize: number;
+  private readonly entries = new Map<string, SenderCacheEntry>();
+
+  constructor(options: SenderCacheOptions = {}) {
+    this.ttlMs = options.ttlMs ?? DEFAULT_TTL_MS;
+    this.maxSize = Math.max(0, options.maxSize ?? DEFAULT_MAX_SIZE);
+  }
+
+  get(openId: string): string | null {
+    const entry = this.entries.get(openId);
+    if (entry === undefined) return null;
+
+    if (entry.expiresAt <= Date.now()) {
+      this.entries.delete(openId);
+      return null;
+    }
+
+    this.entries.delete(openId);
+    this.entries.set(openId, entry);
+    return entry.name;
+  }
+
+  set(openId: string, name: string): void {
+    if (this.maxSize === 0) return;
+
+    this.entries.delete(openId);
+    this.entries.set(openId, { name, expiresAt: Date.now() + this.ttlMs });
+
+    while (this.entries.size > this.maxSize) {
+      const oldest = this.entries.keys().next().value;
+      if (oldest === undefined) break;
+      this.entries.delete(oldest);
+    }
+  }
+
+  async getOrFetch(
+    openId: string,
+    fetchFn: (openId: string) => Promise<string | null>,
+  ): Promise<string | null> {
+    const cached = this.get(openId);
+    if (cached !== null) return cached;
+
+    const fetched = await fetchFn(openId);
+    if (fetched !== null) {
+      this.set(openId, fetched);
+    }
+    return fetched;
+  }
+
+  clear(): void {
+    this.entries.clear();
+  }
+
+  size(): number {
+    this.pruneExpired();
+    return this.entries.size;
+  }
+
+  private pruneExpired(): void {
+    const now = Date.now();
+    for (const [openId, entry] of this.entries) {
+      if (entry.expiresAt <= now) {
+        this.entries.delete(openId);
+      }
+    }
+  }
+}