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/helpers.go

@@ -26,21 +26,10 @@ func requireStringParam(cfg config.SinkConfig, key string) (string, error) {
 	return s, nil
 }
 
-// RegisterPostgresSchemaForConfiguredSinks registers one Postgres schema for each
-// configured sink using driver=postgres.
-func RegisterPostgresSchemaForConfiguredSinks(cfg *config.Config, schema PostgresSchema) error {
-	if cfg == nil {
-		return fmt.Errorf("register postgres schemas: config is nil")
+// PostgresFactory returns a sink factory that builds the built-in Postgres sink
+// using the provided downstream schema definition.
+func PostgresFactory(schema PostgresSchema) Factory {
+	return func(cfg config.SinkConfig) (Sink, error) {
+		return NewPostgresSinkFromConfig(cfg, schema)
 	}
-
-	for i, sk := range cfg.Sinks {
-		if !strings.EqualFold(strings.TrimSpace(sk.Driver), "postgres") {
-			continue
-		}
-		if err := RegisterPostgresSchema(sk.Name, schema); err != nil {
-			return fmt.Errorf("register postgres schema for sinks[%d] name=%q: %w", i, sk.Name, err)
-		}
-	}
-
-	return nil
 }