test(18-01): add failing tests for household timezone accessor + IANA validator

- RED gate: tests for getHouseholdTimezone fallback chain (stored → TZ env → Intl)
- Tests for null row value falling through to TZ env branch
- Tests for isValidIanaTimezone (UTC, Etc/UTC, America/Chicago, Europe/London pass; garbage fails)
- Mock Drizzle select chain follows requireAdmin.test.ts pattern
- Saves/restores process.env.TZ in beforeEach/afterEach to prevent env state leaks

apps/api/tests/lib/householdTimezone.test.ts

@@ -0,0 +1,130 @@
+/**
+ * householdTimezone unit tests (Plan 18-01)
+ *
+ * Behavior-pinned contracts:
+ *   1. getHouseholdTimezone: DB row present → returns stored value
+ *   2. getHouseholdTimezone: no DB row, process.env.TZ set → returns TZ value
+ *   3. getHouseholdTimezone: no DB row, no TZ env var → returns Intl.DateTimeFormat().resolvedOptions().timeZone
+ *   4. getHouseholdTimezone: DB row present but value is null → falls through to env.TZ branch
+ *   5. isValidIanaTimezone: returns true for valid IANA zones ('UTC', 'Etc/UTC', 'America/Chicago', 'Europe/London')
+ *   6. isValidIanaTimezone: returns false for garbage ('Not/AZone', '', 'Mars/Phobos')
+ *
+ * Uses a mocked Drizzle db chain (same pattern as requireAdmin.test.ts).
+ * Saves and restores process.env.TZ around each test to avoid leaking env state.
+ *
+ * Run: pnpm --filter @familysync/api exec vitest run tests/lib/householdTimezone.test.ts
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+
+// Mock the db singleton so tests do not need a live MariaDB connection.
+vi.mock('../../src/db/client.js', () => ({
+  db: {
+    select: vi.fn(),
+  },
+}));
+
+import { db } from '../../src/db/client.js';
+import { getHouseholdTimezone, isValidIanaTimezone } from '../../src/lib/householdTimezone.js';
+
+const mockDb = db as { select: ReturnType<typeof vi.fn> };
+
+// ── DB query chain builder ────────────────────────────────────────────────────
+
+/**
+ * Builds a Drizzle select() → from() → where() → limit() chain
+ * that resolves to the given array.
+ */
+function makeSelectChain(resolvedValue: unknown[]) {
+  const chain = {
+    from: vi.fn(),
+    where: vi.fn(),
+    limit: vi.fn().mockResolvedValue(resolvedValue),
+  };
+  chain.from.mockReturnValue(chain);
+  chain.where.mockReturnValue(chain);
+  return chain;
+}
+
+// ── Test suite ────────────────────────────────────────────────────────────────
+
+describe('getHouseholdTimezone', () => {
+  let originalTZ: string | undefined;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    // Save the current TZ env value before each test
+    originalTZ = process.env.TZ;
+  });
+
+  afterEach(() => {
+    // Restore TZ to its original value (or delete if it was unset)
+    if (originalTZ === undefined) {
+      delete process.env.TZ;
+    } else {
+      process.env.TZ = originalTZ;
+    }
+  });
+
+  it('returns the stored value when the DB has a row for household_timezone', async () => {
+    mockDb.select.mockReturnValue(makeSelectChain([{ value: 'America/Chicago' }]));
+
+    const result = await getHouseholdTimezone(mockDb as never);
+    expect(result).toBe('America/Chicago');
+  });
+
+  it('falls back to process.env.TZ when the DB returns no row', async () => {
+    mockDb.select.mockReturnValue(makeSelectChain([]));
+    process.env.TZ = 'America/New_York';
+
+    const result = await getHouseholdTimezone(mockDb as never);
+    expect(result).toBe('America/New_York');
+  });
+
+  it('falls back to Intl.DateTimeFormat().resolvedOptions().timeZone when no DB row and TZ is unset', async () => {
+    mockDb.select.mockReturnValue(makeSelectChain([]));
+    delete process.env.TZ;
+
+    const expected = Intl.DateTimeFormat().resolvedOptions().timeZone;
+    const result = await getHouseholdTimezone(mockDb as never);
+    expect(result).toBe(expected);
+  });
+
+  it('falls through to process.env.TZ when the DB row value is null', async () => {
+    mockDb.select.mockReturnValue(makeSelectChain([{ value: null }]));
+    process.env.TZ = 'Europe/London';
+
+    const result = await getHouseholdTimezone(mockDb as never);
+    expect(result).toBe('Europe/London');
+  });
+});
+
+describe('isValidIanaTimezone', () => {
+  it("returns true for 'UTC'", () => {
+    expect(isValidIanaTimezone('UTC')).toBe(true);
+  });
+
+  it("returns true for 'Etc/UTC'", () => {
+    expect(isValidIanaTimezone('Etc/UTC')).toBe(true);
+  });
+
+  it("returns true for 'America/Chicago'", () => {
+    expect(isValidIanaTimezone('America/Chicago')).toBe(true);
+  });
+
+  it("returns true for 'Europe/London'", () => {
+    expect(isValidIanaTimezone('Europe/London')).toBe(true);
+  });
+
+  it("returns false for 'Not/AZone'", () => {
+    expect(isValidIanaTimezone('Not/AZone')).toBe(false);
+  });
+
+  it("returns false for empty string ''", () => {
+    expect(isValidIanaTimezone('')).toBe(false);
+  });
+
+  it("returns false for 'Mars/Phobos'", () => {
+    expect(isValidIanaTimezone('Mars/Phobos')).toBe(false);
+  });
+});