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.

config/config_test.go

@@ -12,20 +12,12 @@ func TestSourceConfigExpectedKinds(t *testing.T) {
 		want []string
 	}{
 		{
-			name: "plural kinds preferred",
+			name: "plural kinds normalized",
 			cfg: SourceConfig{
 				Kinds: []string{" observation ", "forecast"},
-				Kind:  "alert",
 			},
 			want: []string{"observation", "forecast"},
 		},
-		{
-			name: "legacy singular fallback",
-			cfg: SourceConfig{
-				Kind: " alert ",
-			},
-			want: []string{"alert"},
-		},
 		{
 			name: "empty kinds",
 			cfg:  SourceConfig{},