test: Add DiffCache unit tests and getPhaseReviewDiff cache integration tests

Creates diff-cache.ts module with generic DiffCache<T> class (TTL, prefix
invalidation, env-var configuration) and exports phaseMetaCache / fileDiffCache
singletons. Wires cache into getPhaseReviewDiff via getHeadCommitHash on
BranchManager. Adds 6 unit tests for DiffCache and 5 integration tests
verifying cache hit/miss behaviour, prefix invalidation, and NOT_FOUND guard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/server/review/diff-cache.test.ts

@@ -0,0 +1,76 @@
+/**
+ * Unit tests for DiffCache class.
+ *
+ * Tests TTL expiry, prefix invalidation, and env-var TTL configuration.
+ */
+
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { DiffCache } from './diff-cache.js';
+
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe('DiffCache', () => {
+  it('returns undefined for a key that was never set', () => {
+    const cache = new DiffCache<string>(5000);
+    expect(cache.get('nonexistent')).toBeUndefined();
+  });
+
+  it('returns value when entry has not expired', () => {
+    vi.spyOn(Date, 'now').mockReturnValue(1000);
+    const cache = new DiffCache<string>(5000);
+    cache.set('key', 'value');
+    vi.spyOn(Date, 'now').mockReturnValue(5999); // 4999ms elapsed, TTL=5000
+    expect(cache.get('key')).toBe('value');
+  });
+
+  it('returns undefined and deletes the entry when TTL has elapsed', () => {
+    vi.spyOn(Date, 'now').mockReturnValue(1000);
+    const cache = new DiffCache<string>(5000);
+    cache.set('key', 'value');
+    vi.spyOn(Date, 'now').mockReturnValue(6001); // 5001ms elapsed, TTL=5000
+    expect(cache.get('key')).toBeUndefined();
+    // Verify the key is no longer stored (second get also returns undefined)
+    vi.spyOn(Date, 'now').mockReturnValue(6001);
+    expect(cache.get('key')).toBeUndefined();
+  });
+
+  it('overwrites an existing entry and resets its TTL', () => {
+    vi.spyOn(Date, 'now').mockReturnValue(1000);
+    const cache = new DiffCache<string>(5000);
+    cache.set('key', 'first');
+    vi.spyOn(Date, 'now').mockReturnValue(4000); // overwrite before expiry
+    cache.set('key', 'second');
+    vi.spyOn(Date, 'now').mockReturnValue(8999); // 4999ms after overwrite, TTL=5000
+    expect(cache.get('key')).toBe('second');
+    vi.spyOn(Date, 'now').mockReturnValue(9001); // 5001ms after overwrite
+    expect(cache.get('key')).toBeUndefined();
+  });
+
+  it('invalidateByPrefix removes all matching keys and preserves others', () => {
+    const cache = new DiffCache<string>(60_000);
+    cache.set('phase-1:abc', 'a');
+    cache.set('phase-1:abc:file.ts', 'b');
+    cache.set('phase-2:xyz', 'c');
+    cache.invalidateByPrefix('phase-1:');
+    expect(cache.get('phase-1:abc')).toBeUndefined();
+    expect(cache.get('phase-1:abc:file.ts')).toBeUndefined();
+    expect(cache.get('phase-2:xyz')).toBe('c');
+  });
+
+  it('singleton instances use REVIEW_DIFF_CACHE_TTL_MS env var for TTL', async () => {
+    vi.resetModules();
+    vi.stubEnv('REVIEW_DIFF_CACHE_TTL_MS', '1000');
+    const { phaseMetaCache } = await import('./diff-cache.js');
+
+    vi.spyOn(Date, 'now').mockReturnValue(0);
+    phaseMetaCache.set('key', {} as any);
+    vi.spyOn(Date, 'now').mockReturnValue(999);
+    expect(phaseMetaCache.get('key')).toBeDefined();
+    vi.spyOn(Date, 'now').mockReturnValue(1001);
+    expect(phaseMetaCache.get('key')).toBeUndefined();
+
+    vi.unstubAllEnvs();
+  });
+});

apps/server/review/diff-cache.ts

@@ -0,0 +1,48 @@
+/**
+ * DiffCache — in-memory TTL cache for git diff results.
+ *
+ * Keyed by `phaseId:headHash` (or `phaseId:headHash:filePath` for per-file diffs).
+ * TTL defaults to 5 minutes, configurable via REVIEW_DIFF_CACHE_TTL_MS env var.
+ * Prefix-based invalidation clears all entries for a phase when a new commit lands.
+ */
+
+interface CacheEntry<T> {
+  value: T;
+  expiresAt: number;
+}
+
+export class DiffCache<T> {
+  private store = new Map<string, CacheEntry<T>>();
+  private ttlMs: number;
+
+  constructor(ttlMs: number) {
+    this.ttlMs = ttlMs;
+  }
+
+  get(key: string): T | undefined {
+    const entry = this.store.get(key);
+    if (!entry) return undefined;
+    if (Date.now() > entry.expiresAt) {
+      this.store.delete(key);
+      return undefined;
+    }
+    return entry.value;
+  }
+
+  set(key: string, value: T): void {
+    this.store.set(key, { value, expiresAt: Date.now() + this.ttlMs });
+  }
+
+  invalidateByPrefix(prefix: string): void {
+    for (const key of this.store.keys()) {
+      if (key.startsWith(prefix)) this.store.delete(key);
+    }
+  }
+}
+
+const TTL = parseInt(process.env.REVIEW_DIFF_CACHE_TTL_MS ?? '300000', 10);
+
+// Typed as unknown here; the actual type constraint is enforced at call sites
+// in phase.ts where PhaseMetaResponse / FileDiffResponse are inferred.
+export const phaseMetaCache = new DiffCache<unknown>(TTL);
+export const fileDiffCache = new DiffCache<unknown>(TTL);