Moved common HTTP polling helpers into feedkit and implemented support for ETag and Last-Modified

doc.go

@@ -50,6 +50,19 @@
 //     Both share Input{Name()}. A source may emit 0..N events per poll/run step,
 //     and may emit multiple event kinds.
 //
+//     For HTTP-backed polling sources, sources.NewHTTPSource provides a shared
+//     helper for generic params:
+//
+//   - params.url
+//
+//   - params.user_agent
+//
+//   - params.conditional (optional, default true)
+//
+//     When conditional polling is enabled, feedkit opportunistically uses ETag
+//     and Last-Modified validators. A 304 Not Modified response is treated as a
+//     successful poll that emits no events.
+//
 //   - scheduler
 //     Runs one goroutine per job:
 //