refactor: collapse feature flags to federation+nats

Dockerfile

@@ -74,11 +74,10 @@ COPY crates ./crates
 # All sqlx queries use the runtime API (no query! macros), so no database
 # or .sqlx cache is needed at compile time.
 #
-# To build with PostgreSQL backend instead:
-#   --build-arg FEATURES=postgres,postgres-federation
-# To add NATS support (EVENT_BUS_BACKEND=nats):
-#   --build-arg FEATURES=sqlite,sqlite-federation,nats
-ARG FEATURES=sqlite,sqlite-federation
+# Features: federation (ActivityPub), nats (EVENT_BUS_BACKEND=nats).
+# Build without federation:  --build-arg FEATURES=
+# To add NATS support:       --build-arg FEATURES=federation,nats
+ARG FEATURES=federation
 RUN cargo build --release -p server -p worker --no-default-features --features "${FEATURES}"
 
 # ----- runtime -----