v0.x: add reusable HTTP source spine; fix routing; upstream HTTP transport helper

- fix dispatch route compilation so empty Kinds matches all (nil), not none
- introduce internal/sources/common/HTTPSource to centralize HTTP polling boilerplate:
  - standard cfg parsing (url + user_agent)
  - default HTTP client + Accept/User-Agent headers
  - consistent error wrapping
- refactor observation sources (nws/openmeteo/openweather) to use HTTPSource
- upstream generic HTTP fetch/limits/timeout helper from weatherfeeder to feedkit:
  - move internal/sources/common/http.go -> feedkit/transport/http.go
  - keep behavior: status checks, max-body limit, default timeout

internal/sources/openweather/observation.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"net/http"
 	"net/url"
 	"strings"
 	"time"
@@ -23,40 +22,32 @@ import (
 // system in the response body. To keep normalization deterministic, this driver *requires*
 // `units=metric`. If absent (or non-metric), the driver returns an error.
 type ObservationSource struct {
-	name      string
-	url       string
-	userAgent string
-	client    *http.Client
+	http *common.HTTPSource
 }
 
 func NewObservationSource(cfg config.SourceConfig) (*ObservationSource, error) {
 	const driver = "openweather_observation"
 
-	c, err := common.RequireHTTPSourceConfig(driver, cfg)
+	hs, err := common.NewHTTPSource(driver, cfg, "application/json")
 	if err != nil {
 		return nil, err
 	}
 
-	if err := requireMetricUnits(c.URL); err != nil {
-		return nil, fmt.Errorf("openweather_observation %q: %w", c.Name, err)
+	if err := requireMetricUnits(hs.URL); err != nil {
+		return nil, fmt.Errorf("%s %q: %w", hs.Driver, hs.Name, err)
 	}
 
-	return &ObservationSource{
-		name:      c.Name,
-		url:       c.URL,
-		userAgent: c.UserAgent,
-		client:    common.NewHTTPClient(common.DefaultHTTPTimeout),
-	}, nil
+	return &ObservationSource{http: hs}, nil
 }
 
-func (s *ObservationSource) Name() string { return s.name }
+func (s *ObservationSource) Name() string { return s.http.Name }
 
 func (s *ObservationSource) Kind() event.Kind { return event.Kind("observation") }
 
 func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 	// Re-check policy defensively (in case the URL is mutated after construction).
-	if err := requireMetricUnits(s.url); err != nil {
-		return nil, fmt.Errorf("openweather_observation %q: %w", s.name, err)
+	if err := requireMetricUnits(s.http.URL); err != nil {
+		return nil, fmt.Errorf("%s %q: %w", s.http.Driver, s.http.Name, err)
 	}
 
 	raw, meta, err := s.fetchRaw(ctx)
@@ -64,9 +55,9 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 		return nil, err
 	}
 
-	eventID := buildEventID(s.name, meta)
+	eventID := buildEventID(s.http.Name, meta)
 	if strings.TrimSpace(eventID) == "" {
-		eventID = fmt.Sprintf("openweather:current:%s:%s", s.name, time.Now().UTC().Format(time.RFC3339Nano))
+		eventID = fmt.Sprintf("openweather:current:%s:%s", s.http.Name, time.Now().UTC().Format(time.RFC3339Nano))
 	}
 
 	var effectiveAt *time.Time
@@ -77,7 +68,7 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 
 	return common.SingleRawEvent(
 		s.Kind(),
-		s.name,
+		s.http.Name,
 		standards.SchemaRawOpenWeatherCurrentV1,
 		eventID,
 		effectiveAt,
@@ -102,15 +93,13 @@ type openWeatherMeta struct {
 }
 
 func (s *ObservationSource) fetchRaw(ctx context.Context) (json.RawMessage, openWeatherMeta, error) {
-	b, err := common.FetchBody(ctx, s.client, s.url, s.userAgent, "application/json")
+	raw, err := s.http.FetchJSON(ctx)
 	if err != nil {
-		return nil, openWeatherMeta{}, fmt.Errorf("openweather_observation %q: %w", s.name, err)
+		return nil, openWeatherMeta{}, err
 	}
 
-	raw := json.RawMessage(b)
-
 	var meta openWeatherMeta
-	if err := json.Unmarshal(b, &meta); err != nil {
+	if err := json.Unmarshal(raw, &meta); err != nil {
 		return raw, openWeatherMeta{}, nil
 	}