sources: standardize HTTP source config + factor raw-event boilerplate

- Require params.user_agent for all HTTP sources (uniform config across providers)
- Add common.RequireHTTPSourceConfig() to validate name/url/user_agent in one call
- Add common.NewHTTPClient() with DefaultHTTPTimeout for consistent client setup
- Add common.SingleRawEvent() to centralize event envelope construction + validation
- Refactor NWS/Open-Meteo/OpenWeather observation sources to use new helpers

internal/sources/common/event.go

@@ -0,0 +1,37 @@
+// FILE: ./internal/sources/common/event.go
+package common
+
+import (
+	"time"
+
+	"gitea.maximumdirect.net/ejr/feedkit/event"
+)
+
+// SingleRawEvent constructs, validates, and returns a slice containing exactly one event.
+//
+// This removes the repetitive "event envelope ceremony" from individual sources.
+// Sources remain responsible for:
+//   - fetching bytes (raw payload)
+//   - choosing Schema (raw schema identifier)
+//   - computing a stable Event.ID and (optional) EffectiveAt
+func SingleRawEvent(kind event.Kind, sourceName string, schema string, id string, effectiveAt *time.Time, payload any) ([]event.Event, error) {
+	e := event.Event{
+		ID:          id,
+		Kind:        kind,
+		Source:      sourceName,
+		EmittedAt:   time.Now().UTC(),
+		EffectiveAt: effectiveAt,
+
+		// RAW schema (normalizer matches on this).
+		Schema: schema,
+
+		// Raw payload (usually json.RawMessage). Normalizer will decode and map to canonical model.
+		Payload: payload,
+	}
+
+	if err := e.Validate(); err != nil {
+		return nil, err
+	}
+
+	return []event.Event{e}, nil
+}