feat(F-004): typed fail-fast config and feature flag module

- loadConfig: pure over env object, accumulates all problems, names var names only
- DATABASE_URL now required at startup; PORT/HOST/LOG_LEVEL/NODE_ENV/REDIS_URL defaulted
- flags module behind FeatureFlagProvider; unknown flags OFF; runtime setEnabled (no redeploy)
- buildApp decorates app.flags; server.ts fail-fast before app boot
- tests caught and fixed flag-store case-normalization bug before gates
- zero new dependencies; all gates approved; verify.sh green

project/src/modules/flags/domain/feature-flag-store.ts

@@ -0,0 +1,34 @@
+/**
+ * Feature flags domain. Deployment and activation are separate operations:
+ * flags can change at runtime without redeploy. Unknown flags are OFF
+ * (fail-safe default for risky paths).
+ */
+
+export interface FeatureFlagProvider {
+  isEnabled(name: string): boolean;
+}
+
+export class InMemoryFeatureFlagStore implements FeatureFlagProvider {
+  private readonly state: Map<string, boolean>;
+
+  constructor(initial: Readonly<Record<string, boolean>> = {}) {
+    this.state = new Map(
+      Object.entries(initial).map(([name, enabled]) => [name.toLowerCase(), enabled]),
+    );
+  }
+
+  isEnabled(name: string): boolean {
+    return this.state.get(name.toLowerCase()) === true;
+  }
+
+  /** Runtime mutation: activation does not require a redeploy. */
+  setEnabled(name: string, enabled: boolean): void {
+    this.state.set(name.toLowerCase(), enabled);
+  }
+}
+
+export function createFlagStore(
+  initial: Readonly<Record<string, boolean>> = {},
+): InMemoryFeatureFlagStore {
+  return new InMemoryFeatureFlagStore(initial);
+}

project/src/modules/flags/index.ts

@@ -0,0 +1,9 @@
+/**
+ * Public API of the flags module. Everything the module exposes to the
+ * outside world goes through this file.
+ */
+export {
+  type FeatureFlagProvider,
+  InMemoryFeatureFlagStore,
+  createFlagStore,
+} from './domain/feature-flag-store.js';

project/src/modules/flags/tests/feature-flag-store.test.ts

@@ -0,0 +1,47 @@
+import { describe, expect, it } from 'vitest';
+import { createFlagStore } from '../index.js';
+
+describe('InMemoryFeatureFlagStore', () => {
+  it('reflects seeded state', () => {
+    const store = createFlagStore({ new_checkout: true, beta_search: false });
+    expect(store.isEnabled('new_checkout')).toBe(true);
+    expect(store.isEnabled('beta_search')).toBe(false);
+  });
+
+  it('defaults unknown flags to OFF (fail-safe)', () => {
+    const store = createFlagStore();
+    expect(store.isEnabled('never_heard_of_it')).toBe(false);
+  });
+
+  it('matches flag names case-insensitively', () => {
+    const store = createFlagStore({ New_Checkout: true });
+    expect(store.isEnabled('new_checkout')).toBe(true);
+    expect(store.isEnabled('NEW_CHECKOUT')).toBe(true);
+  });
+
+  it('flips state at runtime without a new store (no redeploy)', () => {
+    const store = createFlagStore({ risky_path: false });
+    expect(store.isEnabled('risky_path')).toBe(false);
+
+    store.setEnabled('risky_path', true);
+    expect(store.isEnabled('risky_path')).toBe(true);
+
+    store.setEnabled('risky_path', false);
+    expect(store.isEnabled('risky_path')).toBe(false);
+  });
+
+  it('skips a guarded path when the flag is off', () => {
+    const store = createFlagStore({ risky_path: false });
+    let ran = false;
+    if (store.isEnabled('risky_path')) {
+      ran = true;
+    }
+    expect(ran).toBe(false);
+
+    store.setEnabled('risky_path', true);
+    if (store.isEnabled('risky_path')) {
+      ran = true;
+    }
+    expect(ran).toBe(true);
+  });
+});