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_test.go

@@ -15,13 +15,6 @@ type testInput struct {
 
 func (s testInput) Name() string { return s.name }
 
-type testKindSource struct {
-	testInput
-	kind event.Kind
-}
-
-func (s testKindSource) Kind() event.Kind { return s.kind }
-
 type testKindsSource struct {
 	testInput
 	kinds []event.Kind
@@ -29,18 +22,6 @@ type testKindsSource struct {
 
 func (s testKindsSource) Kinds() []event.Kind { return s.kinds }
 
-func TestValidateExpectedKindsLegacyKindFallback(t *testing.T) {
-	cfg := config.SourceConfig{Kind: "observation"}
-	in := testKindSource{
-		testInput: testInput{name: "test"},
-		kind:      event.Kind("observation"),
-	}
-
-	if err := ValidateExpectedKinds(cfg, in); err != nil {
-		t.Fatalf("ValidateExpectedKinds() unexpected error: %v", err)
-	}
-}
-
 func TestValidateExpectedKindsSubsetAllowed(t *testing.T) {
 	cfg := config.SourceConfig{Kinds: []string{"observation"}}
 	in := testKindsSource{