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/media/src/__seeds__/dev.ts

@@ -0,0 +1,38 @@
+import { mediaFactory } from "../__factories__/media.factory.js";
+import type { Media } from "../entities/models/media.js";
+
+/**
+ * Realistic media seed for dev mode + storybook stories.
+ *
+ * Built from `mediaFactory` so factory defaults take care of boring fields
+ * and we only override what makes the data look like a real media library.
+ *
+ * Lazily produced so importing this module is side-effect-free — the
+ * factory's sequence counter only advances when a binder calls
+ * `buildDevMedia()`.
+ */
+export function buildDevMedia(): Media[] {
+  return [
+    mediaFactory.build({
+      id: "placeholder-1",
+      alt: "Hero background image",
+      url: "/dev/placeholder-1.jpg",
+      filename: "placeholder-1.jpg",
+      mimeType: "image/jpeg",
+    }),
+    mediaFactory.build({
+      id: "placeholder-2",
+      alt: "Team photo",
+      url: "/dev/placeholder-2.jpg",
+      filename: "placeholder-2.jpg",
+      mimeType: "image/jpeg",
+    }),
+    mediaFactory.build({
+      id: "placeholder-3",
+      alt: "Product screenshot",
+      url: "/dev/placeholder-3.png",
+      filename: "placeholder-3.png",
+      mimeType: "image/png",
+    }),
+  ];
+}