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/http.go

@@ -1,3 +1,4 @@
+// FILE: ./internal/sources/common/http.go
 package common
 
 import (
@@ -5,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"time"
 )
 
 // maxResponseBodyBytes is a hard safety limit on HTTP response bodies.
@@ -12,6 +14,19 @@ import (
 // or malicious large responses.
 const maxResponseBodyBytes = 2 << 21 // 4 MiB
 
+// DefaultHTTPTimeout is the standard timeout used by weatherfeeder HTTP sources.
+// Individual drivers may override this if they have a specific need.
+const DefaultHTTPTimeout = 10 * time.Second
+
+// NewHTTPClient returns a simple http.Client configured with a timeout.
+// If timeout <= 0, DefaultHTTPTimeout is used.
+func NewHTTPClient(timeout time.Duration) *http.Client {
+	if timeout <= 0 {
+		timeout = DefaultHTTPTimeout
+	}
+	return &http.Client{Timeout: timeout}
+}
+
 func FetchBody(ctx context.Context, client *http.Client, url, userAgent, accept string) ([]byte, error) {
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
 	if err != nil {