feat(features): add bind-dev-seed binders for auth/marketing-pages/navigation/media

Mirrors the canonical blog pattern landed earlier on this branch.

Per feature:
- src/__seeds__/dev.ts — lazy buildDev<Entities>() function using the
  feature's existing factory for sensible defaults
- src/di/bind-dev-seed.ts — bindDevSeed<Feature>() async function that
  rebinds the repo symbol(s) to a populated MockXRepository via
  .toConstantValue
- src/di/bind-dev-seed.test.ts — 3+ tests per feature (populates,
  reachable by id/slug, idempotent)
- package.json — adds ./di/bind-dev-seed subpath export

Tests + use cases continue to construct mocks directly; the seed never
runs from a *.test.ts path. App boot wiring (USE_DEV_SEED env branch)
follows in a separate commit.

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

packages/auth/src/__seeds__/dev.ts

@@ -0,0 +1,27 @@
+import { userFactory } from "../__factories__/user.factory.js";
+import type { User } from "../entities/models/user.js";
+
+/**
+ * Realistic auth seed for dev mode + storybook stories.
+ *
+ * Built from `userFactory` so factory defaults take care of boring fields
+ * and we only override what makes the data look like a real user database.
+ *
+ * Lazily produced so importing this module is side-effect-free — the
+ * factory's sequence counter only advances when a binder calls
+ * `buildDevUsers()`.
+ */
+export function buildDevUsers(): User[] {
+  return [
+    userFactory.build({
+      id: "alice",
+      username: "alice",
+      passwordHash: "hashed_secret_alice",
+    }),
+    userFactory.build({
+      id: "bob",
+      username: "bob",
+      passwordHash: "hashed_secret_bob",
+    }),
+  ];
+}