Add modular app structure for L1 server refactoring

Phase 2 of the full modernization:
- App factory pattern with create_app()
- Settings via dataclass with env vars
- Dependency injection container
- Router stubs for auth, storage, api, recipes, cache, runs
- Service layer stubs for run, recipe, cache
- Repository layer placeholder

Routes are stubs that import from legacy server.py during migration.
Next: Migrate each router fully with templates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routers/__init__.py

@@ -0,0 +1,23 @@
+"""
+L1 Server Routers.
+
+Each router handles a specific domain of functionality.
+"""
+
+from . import auth
+from . import storage
+from . import api
+from . import recipes
+from . import cache
+from . import runs
+from . import home
+
+__all__ = [
+    "auth",
+    "storage",
+    "api",
+    "recipes",
+    "cache",
+    "runs",
+    "home",
+]