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/source.go

@@ -31,9 +31,6 @@ type PollSource interface {
 	Poll(ctx context.Context) ([]event.Event, error)
 }
 
-// Source is a compatibility alias for the legacy polling-source name.
-type Source = PollSource
-
 // StreamSource is an event-driven source (NATS/RabbitMQ/MQTT/etc).
 //
 // Run should block, producing events into `out` until ctx is cancelled or a fatal error occurs.
@@ -43,12 +40,6 @@ type StreamSource interface {
 	Run(ctx context.Context, out chan<- event.Event) error
 }
 
-// KindSource is an optional interface for sources that advertise one "primary" kind.
-// This is legacy-friendly but no longer required.
-type KindSource interface {
-	Kind() event.Kind
-}
-
 // KindsSource is an optional interface for sources that advertise multiple kinds.
 type KindsSource interface {
 	Kinds() []event.Kind