Refactored the scheduler and source interfaces to accommondate both polling (e.g., HTTP) sources and streaming (e.g., message queue) sources.

config/load.go

@@ -83,9 +83,9 @@ func (c *Config) Validate() error {
 				m.Add(fieldErr(path+".driver", "is required (e.g. openmeteo_observation, rss_feed, ...)"))
 			}
 
-			// Every
-			if s.Every.Duration <= 0 {
-				m.Add(fieldErr(path+".every", "must be a positive duration (e.g. 15m, 1m, 30s)"))
+			// Every (optional but if present must be >=0)
+			if s.Every.Duration < 0 {
+				m.Add(fieldErr(path+".every", "is optional, but must be a positive duration (e.g. 15m, 1m, 30s) if provided"))
 			}
 
 			// Kind (optional but if present must be non-empty after trimming)