fix(auth): typed context keys and real sentinel errors

backend/api/middleware/context.go (new):
- Introduces an unexported ctxKey type and three constants
  (ctxKeyUserAddress, ctxKeyUserTrack, ctxKeyAuthenticated) that
  replace the bare string keys 'user_address', 'user_track', and
  'authenticated'. Bare strings trigger go vet's SA1029 and collide
  with keys from any other package that happens to share the name.
- Helpers: ContextWithAuth, UserAddress, UserTrack, IsAuthenticated.
- Sentinel: ErrMissingAuthorization replaces the misuse of
  http.ErrMissingFile as an auth-missing signal. (http.ErrMissingFile
  belongs to multipart form parsing and was semantically wrong.)

backend/api/middleware/auth.go:
- RequireAuth, OptionalAuth, RequireTrack now all read/write via the
  helpers; no more string literals for context keys in this file.
- extractAuth returns ErrMissingAuthorization instead of
  http.ErrMissingFile.
- Dropped now-unused 'context' import.

backend/api/track4/operator_scripts.go, backend/api/track4/endpoints.go,
backend/api/rest/features.go:
- Read user address / track via middleware.UserAddress() and
  middleware.UserTrack() instead of a raw context lookup with a bare
  string key.
- Import 'github.com/explorer/backend/api/middleware'.

backend/api/track4/operator_scripts_test.go:
- Four test fixtures updated to seed the request context through
  middleware.ContextWithAuth (track 4, authenticated) instead of
  context.WithValue with a bare 'user_address' string. This is the
  load-bearing change that proves typed keys are required: a bare
  string key no longer wakes up the middleware helpers.

backend/api/middleware/context_test.go (new):
- Round-trip test for ContextWithAuth + UserAddress + UserTrack +
  IsAuthenticated.
- Defaults: UserTrack=1, UserAddress="", IsAuthenticated=false on a
  bare context.
- TestContextKeyIsolation: an outside caller that inserts
  'user_address' as a bare string key must NOT be visible to
  UserAddress; proves the type discipline.
- ErrMissingAuthorization sentinel smoke test.

Verification:
- go build ./... clean.
- go vet ./... clean (removes SA1029 on the old bare keys).
- go test ./api/middleware/... ./api/track4/... ./api/rest/... PASS.

Advances completion criterion 3 (Auth correctness).

backend/api/rest/features.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"net/http"
 
+	"github.com/explorer/backend/api/middleware"
 	"github.com/explorer/backend/featureflags"
 )
 
@@ -16,11 +17,8 @@ func (s *Server) handleFeatures(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Extract user track from context (set by auth middleware)
-	// Default to Track 1 (public) if not authenticated
-	userTrack := 1
-	if track, ok := r.Context().Value("user_track").(int); ok {
-		userTrack = track
-	}
+	// Default to Track 1 (public) if not authenticated (handled by helper).
+	userTrack := middleware.UserTrack(r.Context())
 
 	// Get enabled features for this track
 	enabledFeatures := featureflags.GetEnabledFeatures(userTrack)