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/middleware/auth.go

@@ -1,7 +1,6 @@
 package middleware
 
 import (
-	"context"
 	"fmt"
 	"net/http"
 	"strings"
@@ -31,11 +30,7 @@ func (m *AuthMiddleware) RequireAuth(next http.Handler) http.Handler {
 			return
 		}
 
-		// Add user context
-		ctx := context.WithValue(r.Context(), "user_address", address)
-		ctx = context.WithValue(ctx, "user_track", track)
-		ctx = context.WithValue(ctx, "authenticated", true)
-
+		ctx := ContextWithAuth(r.Context(), address, track, true)
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})
 }
@@ -44,11 +39,7 @@ func (m *AuthMiddleware) RequireAuth(next http.Handler) http.Handler {
 func (m *AuthMiddleware) RequireTrack(requiredTrack int) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			// Extract track from context (set by RequireAuth or OptionalAuth)
-			track, ok := r.Context().Value("user_track").(int)
-			if !ok {
-				track = 1 // Default to Track 1 (public)
-			}
+			track := UserTrack(r.Context())
 
 			if !featureflags.HasAccess(track, requiredTrack) {
 				writeForbidden(w, requiredTrack)
@@ -65,40 +56,33 @@ func (m *AuthMiddleware) OptionalAuth(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		address, track, err := m.extractAuth(r)
 		if err != nil {
-			// No auth provided, default to Track 1 (public)
-			ctx := context.WithValue(r.Context(), "user_address", "")
-			ctx = context.WithValue(ctx, "user_track", 1)
-			ctx = context.WithValue(ctx, "authenticated", false)
+			// No auth provided (or auth failed) — fall back to Track 1.
+			ctx := ContextWithAuth(r.Context(), "", defaultTrackLevel, false)
 			next.ServeHTTP(w, r.WithContext(ctx))
 			return
 		}
 
-		// Auth provided, add user context
-		ctx := context.WithValue(r.Context(), "user_address", address)
-		ctx = context.WithValue(ctx, "user_track", track)
-		ctx = context.WithValue(ctx, "authenticated", true)
-
+		ctx := ContextWithAuth(r.Context(), address, track, true)
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})
 }
 
-// extractAuth extracts authentication information from request
+// extractAuth extracts authentication information from the request.
+// Returns ErrMissingAuthorization when no usable Bearer token is present;
+// otherwise returns the error from JWT validation.
 func (m *AuthMiddleware) extractAuth(r *http.Request) (string, int, error) {
-	// Get Authorization header
 	authHeader := r.Header.Get("Authorization")
 	if authHeader == "" {
-		return "", 0, http.ErrMissingFile
+		return "", 0, ErrMissingAuthorization
 	}
 
-	// Check for Bearer token
 	parts := strings.Split(authHeader, " ")
 	if len(parts) != 2 || parts[0] != "Bearer" {
-		return "", 0, http.ErrMissingFile
+		return "", 0, ErrMissingAuthorization
 	}
 
 	token := parts[1]
 
-	// Validate JWT token
 	address, track, err := m.walletAuth.ValidateJWT(token)
 	if err != nil {
 		return "", 0, err