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/package.json

@@ -8,7 +8,8 @@
     "./ui": "./src/ui/index.ts",
     "./cms": "./src/integrations/cms/index.ts",
     "./api": "./src/integrations/api/router.ts",
-    "./di/bind-production": "./src/di/bind-production.ts"
+    "./di/bind-production": "./src/di/bind-production.ts",
+    "./di/bind-dev-seed": "./src/di/bind-dev-seed.ts"
   },
   "scripts": {
     "build": "tsc --noEmit",