Add modular app structure for L2 server

- Create app factory with routers and templates
- Auth, assets, activities, anchors, storage, users, renderers routers
- Federation router for WebFinger and nodeinfo
- Jinja2 templates for L2 pages
- Config and dependency injection

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/__init__.py

@@ -0,0 +1,77 @@
+"""
+Art-DAG L2 Server Application Factory.
+
+Creates and configures the FastAPI application with all routers and middleware.
+"""
+
+from pathlib import Path
+from contextlib import asynccontextmanager
+from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse, HTMLResponse
+
+from artdag_common import create_jinja_env
+
+from .config import settings
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    """Manage database connection pool lifecycle."""
+    import db
+    await db.init_pool()
+    yield
+    await db.close_pool()
+
+
+def create_app() -> FastAPI:
+    """
+    Create and configure the L2 FastAPI application.
+
+    Returns:
+        Configured FastAPI instance
+    """
+    app = FastAPI(
+        title="Art-DAG L2 Server",
+        description="ActivityPub server for Art-DAG ownership and federation",
+        version="1.0.0",
+        lifespan=lifespan,
+    )
+
+    # Initialize Jinja2 templates
+    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,
+            )
+        return JSONResponse({"detail": "Not found"}, status_code=404)
+
+    # Include routers
+    from .routers import auth, assets, activities, anchors, storage, users, renderers
+
+    # Root routes
+    app.include_router(auth.router, prefix="/auth", tags=["auth"])
+    app.include_router(users.router, tags=["users"])
+
+    # Feature routers
+    app.include_router(assets.router, prefix="/assets", tags=["assets"])
+    app.include_router(activities.router, prefix="/activities", tags=["activities"])
+    app.include_router(anchors.router, prefix="/anchors", tags=["anchors"])
+    app.include_router(storage.router, prefix="/storage", tags=["storage"])
+    app.include_router(renderers.router, prefix="/renderers", tags=["renderers"])
+
+    # WebFinger and ActivityPub discovery
+    from .routers import federation
+    app.include_router(federation.router, tags=["federation"])
+
+    return app
+
+
+# Create the default app instance
+app = create_app()