feat: bootstrap backend MVP and architecture docs

backend/app/main.py

@@ -0,0 +1,15 @@
+from fastapi import FastAPI
+
+from app.api.admin import router as admin_router
+from app.api.internal import router as internal_router
+
+app = FastAPI(title="memberapi.ose.tw", version="0.1.0")
+
+
+@app.get("/healthz", tags=["health"])
+def healthz() -> dict[str, str]:
+    return {"status": "ok"}
+
+
+app.include_router(internal_router)
+app.include_router(admin_router)