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/runs.py

@@ -0,0 +1,57 @@
+"""
+Run management routes for L1 server.
+
+Handles run creation, status, listing, and detail views.
+"""
+
+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, get_current_user
+
+router = APIRouter()
+
+
+# TODO: Migrate routes from server.py lines 675-1789
+# - POST /runs - Create run
+# - GET /runs/{run_id} - Get run status
+# - DELETE /runs/{run_id} - Delete run
+# - GET /runs - List runs
+# - GET /run/{run_id} - Run detail page
+# - GET /run/{run_id}/plan - Plan visualization
+# - GET /run/{run_id}/analysis - Analysis results
+# - GET /run/{run_id}/artifacts - Artifacts list
+
+
+@router.get("/runs")
+async def list_runs(
+    request: Request,
+    user: UserContext = Depends(require_auth),
+):
+    """List all runs for the current user."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")
+
+
+@router.get("/run/{run_id}")
+async def run_detail(
+    run_id: str,
+    request: Request,
+    user: UserContext = Depends(require_auth),
+):
+    """Run detail page with tabs for plan/analysis/artifacts."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")
+
+
+@router.get("/run/{run_id}/plan")
+async def run_plan(
+    run_id: str,
+    request: Request,
+    user: UserContext = Depends(require_auth),
+):
+    """Plan visualization as interactive DAG."""
+    # TODO: Implement
+    raise HTTPException(501, "Not yet migrated")