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/nws/observation.go

@@ -5,7 +5,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"net/http"
 	"strings"
 	"time"
 
@@ -21,29 +20,21 @@ import (
 //
 //	https://api.weather.gov/stations/KSTL/observations/latest
 type ObservationSource struct {
-	name      string
-	url       string
-	userAgent string
-	client    *http.Client
+	http *common.HTTPSource
 }
 
 func NewObservationSource(cfg config.SourceConfig) (*ObservationSource, error) {
 	const driver = "nws_observation"
 
-	c, err := common.RequireHTTPSourceConfig(driver, cfg)
+	hs, err := common.NewHTTPSource(driver, cfg, "application/geo+json, application/json")
 	if err != nil {
 		return nil, 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 }
 
 // Kind is used for routing/policy.
 // We keep Kind canonical (observation) even for raw events; Schema differentiates raw vs canonical.
@@ -65,11 +56,13 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 		if ts.IsZero() {
 			ts = time.Now().UTC()
 		}
+
 		station := strings.TrimSpace(meta.StationID)
 		if station == "" {
 			station = "UNKNOWN"
 		}
-		eventID = fmt.Sprintf("nws:observation:%s:%s:%s", s.name, station, ts.UTC().Format(time.RFC3339Nano))
+
+		eventID = fmt.Sprintf("nws:observation:%s:%s:%s", s.http.Name, station, ts.UTC().Format(time.RFC3339Nano))
 	}
 
 	// EffectiveAt is optional; for observations it’s naturally the observation timestamp.
@@ -81,7 +74,7 @@ func (s *ObservationSource) Poll(ctx context.Context) ([]event.Event, error) {
 
 	return common.SingleRawEvent(
 		s.Kind(),
-		s.name,
+		s.http.Name,
 		standards.SchemaRawNWSObservationV1,
 		eventID,
 		effectiveAt,
@@ -106,15 +99,13 @@ type observationMeta struct {
 }
 
 func (s *ObservationSource) fetchRaw(ctx context.Context) (json.RawMessage, observationMeta, error) {
-	b, err := common.FetchBody(ctx, s.client, s.url, s.userAgent, "application/geo+json, application/json")
+	raw, err := s.http.FetchJSON(ctx)
 	if err != nil {
-		return nil, observationMeta{}, fmt.Errorf("nws_observation %q: %w", s.name, err)
+		return nil, observationMeta{}, err
 	}
 
-	raw := json.RawMessage(b)
-
 	var meta observationMeta
-	if err := json.Unmarshal(b, &meta); err != nil {
+	if err := json.Unmarshal(raw, &meta); err != nil {
 		// If metadata decode fails, still return raw; envelope will fall back to computed ID.
 		return raw, observationMeta{}, nil
 	}