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/doc.go

@@ -25,6 +25,9 @@
 //   - params.url
 //   - params.user_agent
 //   - params.conditional (optional, default true)
+//   - params.http_timeout (optional, default transport.DefaultHTTPTimeout)
+//   - params.http_response_body_limit_bytes (optional, default
+//     transport.DefaultHTTPResponseBodyLimitBytes)
 //
 // When validators are available, NewHTTPSource prefers ETag/If-None-Match and
 // falls back to Last-Modified/If-Modified-Since. A 304 Not Modified response is