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/config.go

@@ -0,0 +1,104 @@
+// FILE: ./internal/sources/common/config.go
+package common
+
+import (
+	"fmt"
+	"strings"
+
+	"gitea.maximumdirect.net/ejr/feedkit/config"
+)
+
+// This file centralizes small, boring config-validation patterns shared across
+// weatherfeeder source drivers.
+//
+// Goal: keep driver constructors (New*Source) easy to read and consistent, while
+// keeping driver-specific options in cfg.Params (feedkit remains domain-agnostic).
+
+// HTTPSourceConfig is the standard "HTTP-polling source" config shape used across drivers.
+type HTTPSourceConfig struct {
+	Name      string
+	URL       string
+	UserAgent string
+}
+
+// RequireHTTPSourceConfig enforces weatherfeeder's standard HTTP source config:
+//
+//   - cfg.Name must be present
+//   - cfg.Params must be present
+//   - params.url must be present (accepts "url" or "URL")
+//   - params.user_agent must be present (accepts "user_agent" or "userAgent")
+//
+// We intentionally require a User-Agent for *all* sources, even when upstreams
+// do not strictly require one. This keeps config uniform across providers.
+func RequireHTTPSourceConfig(driver string, cfg config.SourceConfig) (HTTPSourceConfig, error) {
+	if strings.TrimSpace(cfg.Name) == "" {
+		return HTTPSourceConfig{}, fmt.Errorf("%s: name is required", driver)
+	}
+	if cfg.Params == nil {
+		return HTTPSourceConfig{}, fmt.Errorf("%s %q: params are required (need params.url and params.user_agent)", driver, cfg.Name)
+	}
+
+	url, ok := cfg.ParamString("url", "URL")
+	if !ok {
+		return HTTPSourceConfig{}, fmt.Errorf("%s %q: params.url is required", driver, cfg.Name)
+	}
+
+	ua, ok := cfg.ParamString("user_agent", "userAgent")
+	if !ok {
+		return HTTPSourceConfig{}, fmt.Errorf("%s %q: params.user_agent is required", driver, cfg.Name)
+	}
+
+	return HTTPSourceConfig{
+		Name:      cfg.Name,
+		URL:       url,
+		UserAgent: ua,
+	}, nil
+}
+
+// --- The helpers below remain useful for future drivers; they are not required
+// --- by the observation sources after adopting RequireHTTPSourceConfig.
+
+// RequireName ensures cfg.Name is present and non-whitespace.
+func RequireName(driver string, cfg config.SourceConfig) error {
+	if strings.TrimSpace(cfg.Name) == "" {
+		return fmt.Errorf("%s: name is required", driver)
+	}
+	return nil
+}
+
+// RequireParams ensures cfg.Params is non-nil. The "want" string should be a short
+// description of required keys, e.g. "need params.url and params.user_agent".
+func RequireParams(driver string, cfg config.SourceConfig, want string) error {
+	if cfg.Params == nil {
+		return fmt.Errorf("%s %q: params are required (%s)", driver, cfg.Name, want)
+	}
+	return nil
+}
+
+// RequireURL returns the configured URL for a source.
+// Canonical key is "url"; we also accept "URL" as a convenience.
+func RequireURL(driver string, cfg config.SourceConfig) (string, error) {
+	if cfg.Params == nil {
+		return "", fmt.Errorf("%s %q: params are required (need params.url)", driver, cfg.Name)
+	}
+
+	u, ok := cfg.ParamString("url", "URL")
+	if !ok {
+		return "", fmt.Errorf("%s %q: params.url is required", driver, cfg.Name)
+	}
+	return u, nil
+}
+
+// RequireUserAgent returns the configured User-Agent for a source.
+// Canonical key is "user_agent"; we also accept "userAgent" as a convenience.
+func RequireUserAgent(driver string, cfg config.SourceConfig) (string, error) {
+	if cfg.Params == nil {
+		return "", fmt.Errorf("%s %q: params are required (need params.user_agent)", driver, cfg.Name)
+	}
+
+	ua, ok := cfg.ParamString("user_agent", "userAgent")
+	if !ok {
+		return "", fmt.Errorf("%s %q: params.user_agent is required", driver, cfg.Name)
+	}
+	return ua, nil
+}