step 1 complete: runnable FastAPI skeleton

- Full directory structure with all __init__.py stubs
- requirements.txt with all pinned dependencies
- app/config/settings.py (BaseSettings, env-based)
- app/main.py (CORS, lifespan, /api/v1/health)
- Dockerfile (multi-stage, Python 3.12-slim, non-root user)
- docker-compose.yml (app + postgres:16 with healthcheck)
- .env.example
- BACKEND_PLAN.md: mark step 1 done, add one-step-at-a-time rule

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/main.py

@@ -0,0 +1,52 @@
+from contextlib import asynccontextmanager
+
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+from app.config.settings import settings
+
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    # Startup: initialise DB connection pool and agent registry
+    from app.core.agent_registry import registry  # noqa: F401 — triggers module load
+    import app.agents  # noqa: F401 — triggers @registry.register decorators
+
+    yield
+
+    # Shutdown: nothing to clean up for now
+
+
+def create_app() -> FastAPI:
+    app = FastAPI(
+        title="Adiuva Cloud API",
+        version="0.1.0",
+        docs_url="/docs" if settings.ENV == "dev" else None,
+        redoc_url=None,
+        lifespan=lifespan,
+    )
+
+    app.add_middleware(
+        CORSMiddleware,
+        allow_origins=settings.CORS_ORIGINS,
+        allow_credentials=True,
+        allow_methods=["*"],
+        allow_headers=["*"],
+    )
+
+    # Routers (registered when implemented)
+    # from app.api.routes import auth, chat, plans, backup, billing
+    # app.include_router(auth.router, prefix="/api/v1")
+    # app.include_router(chat.router, prefix="/api/v1")
+    # app.include_router(plans.router, prefix="/api/v1")
+    # app.include_router(backup.router, prefix="/api/v1")
+    # app.include_router(billing.router, prefix="/api/v1")
+
+    @app.get("/api/v1/health", tags=["health"])
+    async def health() -> dict:
+        return {"status": "ok", "version": app.version}
+
+    return app
+
+
+app = create_app()