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.

README.md

@@ -86,7 +86,7 @@ The major packages are:
 - `processors/normalize`: built-in normalization processor and helpers
 - `pipeline`: optional processor chain
 - `dispatch`: route compilation and fanout
-- `sinks`: sink interfaces, built-ins, and Postgres registration helpers
+- `sinks`: sink interfaces, built-ins, and explicit Postgres factory helpers
 
 The root package docs in `doc.go` provide a concise package-by-package map for
 Go documentation consumers.