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/load.go

@@ -105,13 +105,7 @@ func (c *Config) Validate() error {
 				}
 			}
 
-			// Kind/Kinds (optional)
-			if s.Kind != "" && len(s.Kinds) > 0 {
-				m.Add(fieldErr(path+".kind", `cannot be set when "kinds" is provided (use only "kinds")`))
-			}
-			if s.Kind != "" && strings.TrimSpace(s.Kind) == "" {
-				m.Add(fieldErr(path+".kind", "cannot be blank (omit it entirely, or provide a non-empty string)"))
-			}
+			// Kinds (optional)
 			for j, k := range s.Kinds {
 				kpath := fmt.Sprintf("%s.kinds[%d]", path, j)
 				if strings.TrimSpace(k) == "" {