Add 404 handler and template

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

app/__init__.py

@@ -6,6 +6,7 @@ Creates and configures the FastAPI application with all routers and middleware.
 
 from pathlib import Path
 from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse
 from fastapi.staticfiles import StaticFiles
 
 from artdag_common import create_jinja_env
@@ -41,6 +42,18 @@ def create_app() -> FastAPI:
     template_dir = Path(__file__).parent / "templates"
     app.state.templates = create_jinja_env(template_dir)
 
+    # Custom 404 handler
+    @app.exception_handler(404)
+    async def not_found_handler(request: Request, exc):
+        from artdag_common.middleware import wants_html
+        if wants_html(request):
+            from artdag_common import render
+            return render(app.state.templates, "404.html", request,
+                user=None,
+                status_code=404,
+            )
+        return JSONResponse({"detail": "Not found"}, status_code=404)
+
     # Include routers
     from .routers import auth, storage, api, recipes, cache, runs, home