feat(sources)!: split source contracts into PollSource/StreamSource and add mode-aware source config

- Introduce explicit source interfaces: sources.PollSource and sources.StreamSource, with shared sources.Input (Name() only).
- Remove mandatory Kind() from the base source contract to support sources that emit multiple kinds.
- Add config.SourceMode (poll, stream, or omitted/auto) and SourceConfig.Kinds (plural expected kinds), while keeping legacy SourceConfig.Kind for compatibility.
- Enforce mode semantics in config validation (poll requires every, stream forbids every) and detect mode/driver mismatches in sources.Registry.
- Update docs and tests for the new source model and config behavior.

config/load.go

@@ -83,15 +83,41 @@ func (c *Config) Validate() error {
 				m.Add(fieldErr(path+".driver", "is required (e.g. openmeteo_observation, rss_feed, ...)"))
 			}
 
-			// 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"))
+			// Mode
+			mode := s.Mode.Normalize()
+			if s.Mode != SourceModeAuto && mode != SourceModePoll && mode != SourceModeStream {
+				m.Add(fieldErr(path+".mode", `must be one of: "poll", "stream" (or omit for auto)`))
 			}
 
-			// Kind (optional but if present must be non-empty after trimming)
+			// Every
+			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"))
+			} else {
+				switch mode {
+				case SourceModePoll:
+					if s.Every.Duration <= 0 {
+						m.Add(fieldErr(path+".every", `is required when mode="poll" (e.g. 15m, 1m, 30s)`))
+					}
+				case SourceModeStream:
+					if s.Every.Duration > 0 {
+						m.Add(fieldErr(path+".every", `must be omitted when mode="stream"`))
+					}
+				}
+			}
+
+			// Kind/Kinds (optional)
+			if s.Kind != "" && len(s.Kinds) > 0 {
+				m.Add(fieldErr(path+".kind", `cannot be set when "kinds" is provided (use only "kinds")`))
+			}
 			if s.Kind != "" && strings.TrimSpace(s.Kind) == "" {
 				m.Add(fieldErr(path+".kind", "cannot be blank (omit it entirely, or provide a non-empty string)"))
 			}
+			for j, k := range s.Kinds {
+				kpath := fmt.Sprintf("%s.kinds[%d]", path, j)
+				if strings.TrimSpace(k) == "" {
+					m.Add(fieldErr(kpath, "kind cannot be empty"))
+				}
+			}
 
 			// Params can be nil; that's fine.
 		}