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.

sources/helpers.go

@@ -127,11 +127,6 @@ func advertisedSourceKinds(in Input) map[event.Kind]bool {
 		return kinds
 	}
 
-	if ks, ok := in.(KindSource); ok {
-		kinds[ks.Kind()] = true
-		return kinds
-	}
-
 	return nil
 }