feat(backend): webhooks deliver signed alerts from an outbox with retries and a test send

backend/src/api/app.py

@@ -38,7 +38,9 @@ from api.routers import (
     storage,
     stories,
     watches,
+    webhooks,
 )
+from api.webhooks import outbox
 from dependencies.container import container
 from utils.cache import DAY_HEADERS, IMMUTABLE_HEADERS, NO_STORE_HEADERS
 from utils.env import env
@@ -58,11 +60,14 @@ mcp_app = mcp.http_app(path="/")
 @asynccontextmanager
 async def lifespan(app_: Starlette) -> AsyncGenerator[None]:
     pool = await container.get(asyncpg.Pool)
+    hub.alert_listeners.append(outbox.wake)
     await hub.start(pool)
     purger = asyncio.create_task(
         purge_loop(pool, await container.get(ContentAddressedStorage))
     )
+    deliverer = asyncio.create_task(outbox.run(pool))
     yield
+    deliverer.cancel()
     purger.cancel()
     await hub.stop()
     await app_.state.dishka_container.close()
@@ -103,6 +108,7 @@ app.include_router(peers.router)
 app.include_router(discover.router)
 app.include_router(annotations.router)
 app.include_router(watches.router)
+app.include_router(webhooks.router)
 app.include_router(shares.router)
 app.include_router(storage.router)
 app.include_router(files.router)