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/recipes.py

@@ -0,0 +1,47 @@
+"""
+Recipe management routes for L1 server.
+
+Handles recipe upload, listing, viewing, and execution.
+"""
+
+from fastapi import APIRouter, Request, Depends, HTTPException
+from fastapi.responses import HTMLResponse
+
+from artdag_common.middleware import UserContext, wants_html
+
+from ..dependencies import require_auth, get_templates
+
+router = APIRouter()
+
+
+# TODO: Migrate routes from server.py lines 1990-2767
+# - POST /recipes/upload - Upload recipe YAML
+# - GET /recipes - List recipes
+# - GET /recipes/{recipe_id} - Get recipe details
+# - DELETE /recipes/{recipe_id} - Delete recipe
+# - POST /recipes/{recipe_id}/run - Run recipe
+# - GET /recipe/{recipe_id} - Recipe detail page
+# - GET /recipe/{recipe_id}/dag - Recipe DAG visualization
+# - POST /ui/recipes/{recipe_id}/run - Run from UI
+# - GET /ui/recipes-list - Recipes list UI
+
+
+@router.get("/recipes")
+async def list_recipes(
+    request: Request,
+    user: UserContext = Depends(require_auth),
+):
+    """List available recipes."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")
+
+
+@router.get("/recipe/{recipe_id}")
+async def recipe_detail(
+    recipe_id: str,
+    request: Request,
+    user: UserContext = Depends(require_auth),
+):
+    """Recipe detail page with DAG visualization."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")