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/registry_test.go

@@ -112,15 +112,15 @@ func TestRegisterBuiltinsExposesExpectedDrivers(t *testing.T) {
 	r := NewRegistry()
 	RegisterBuiltins(r)
 
-	if len(r.byDriver) != 3 {
-		t.Fatalf("len(byDriver) = %d, want 3", len(r.byDriver))
+	if len(r.byDriver) != 2 {
+		t.Fatalf("len(byDriver) = %d, want 2", len(r.byDriver))
 	}
-	for _, driver := range []string{"stdout", "nats", "postgres"} {
+	for _, driver := range []string{"stdout", "nats"} {
 		if _, ok := r.byDriver[driver]; !ok {
 			t.Fatalf("builtins missing driver %q", driver)
 		}
 	}
-	if _, ok := r.byDriver["file"]; ok {
-		t.Fatalf("builtins unexpectedly registered file driver")
+	if _, ok := r.byDriver["postgres"]; ok {
+		t.Fatalf("builtins unexpectedly registered postgres driver")
 	}
 }