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/api.py

@@ -0,0 +1,40 @@
+"""
+3-phase API routes for L1 server.
+
+Provides the plan/execute/run-recipe endpoints for programmatic access.
+"""
+
+from fastapi import APIRouter, Request, Depends, HTTPException
+
+from artdag_common.models.requests import PlanRequest, ExecutePlanRequest, RecipeRunRequest
+
+from ..dependencies import require_auth
+
+router = APIRouter()
+
+
+# TODO: Migrate routes from server.py lines 6036-6241
+# - POST /plan - Generate execution plan
+# - POST /execute - Execute a plan
+# - POST /run-recipe - Run complete recipe
+
+
+@router.post("/plan")
+async def generate_plan(request: PlanRequest):
+    """Generate an execution plan from recipe without executing."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")
+
+
+@router.post("/execute")
+async def execute_plan(request: ExecutePlanRequest):
+    """Execute a previously generated plan."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")
+
+
+@router.post("/run-recipe")
+async def run_recipe(request: RecipeRunRequest):
+    """Run a complete recipe through all 3 phases."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")