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.

sinks/doc.go

@@ -4,16 +4,16 @@
 // External API surface:
 //   - Sink: adapter interface that consumes event.Event values
 //   - Registry / NewRegistry: named sink factory registry
-//   - RegisterBuiltins: registers the built-in sink drivers in this binary
+//   - RegisterBuiltins: registers the schema-free built-in sink drivers
 //
-// Built-in drivers:
+// Built-in sink implementations:
 //   - stdout
 //   - nats
 //   - postgres
 //
 // Optional helpers from helpers.go:
-//   - RegisterPostgresSchemaForConfiguredSinks: registers one Postgres schema
-//     for each configured sink using driver=postgres
+//   - PostgresFactory: returns a sink factory for the built-in Postgres sink
+//     using a provided downstream schema
 //
 // # NATS built-in overview
 //
@@ -59,7 +59,9 @@
 //
 // Example downstream wiring:
 //
-//	sinks.MustRegisterPostgresSchema("pg_main", sinks.PostgresSchema{
+//	sinkReg := sinks.NewRegistry()
+//	sinks.RegisterBuiltins(sinkReg)
+//	sinkReg.Register("postgres", sinks.PostgresFactory(sinks.PostgresSchema{
 //	    Tables: []sinks.PostgresTable{
 //	        {
 //	            Name: "events",
@@ -88,7 +90,7 @@
 //	            },
 //	        }, nil
 //	    },
-//	})
+//	}))
 //
 // Manual pruning via type assertion (administrative helpers):
 //