Refactor feedkit boundaries ahead of v1

Remove global Postgres schema registration in favor of explicit schema-aware sink factory wiring, and update weatherfeeder to register the Postgres sink explicitly. Add optional per-source HTTP timeout and response body limit overrides while keeping feedkit defaults. Remove remaining legacy source/config compatibility surfaces, including singular kind support and old source registry/type aliases, and migrate weatherfeeder sources to plural `Kinds()` metadata. Clean up related docs, tests, and sample config to match the new Postgres, HTTP, and NATS configuration model.

sinks/builtins.go

@@ -12,11 +12,6 @@ func RegisterBuiltins(r *Registry) {
 		return NewStdoutSink(cfg.Name), nil
 	})
 
-	// Postgres sink: persists events durably.
-	r.Register("postgres", func(cfg config.SinkConfig) (Sink, error) {
-		return NewPostgresSinkFromConfig(cfg)
-	})
-
 	// NATS sink: publishes events to a broker for downstream consumers.
 	r.Register("nats", func(cfg config.SinkConfig) (Sink, error) {
 		return NewNATSSinkFromConfig(cfg)