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/storage.py

@@ -0,0 +1,35 @@
+"""
+Storage provider routes for L1 server.
+
+Manages user storage backends (Pinata, web3.storage, local, etc.)
+"""
+
+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 5473-5761
+# - GET /storage - List storage providers
+# - POST /storage - Add storage provider
+# - POST /storage/add - Add via form
+# - GET /storage/{storage_id} - Get storage details
+# - PATCH /storage/{storage_id} - Update storage
+# - DELETE /storage/{storage_id} - Delete storage
+# - POST /storage/{storage_id}/test - Test connection
+# - GET /storage/type/{provider_type} - Get provider config
+
+
+@router.get("")
+async def list_storage(
+    request: Request,
+    user: UserContext = Depends(require_auth),
+):
+    """List user's storage providers."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")