Add JWT authentication

- Create register and login use cases
- Implement JWT service for token generation and validation
- Add authentication middleware to protect habit endpoints
- Create auth HTTP handlers (register, login)
- Update habit handlers to extract userID from JWT token
- Register/login endpoints: POST /auth/register, POST /auth/login
- Habit endpoints now require Bearer token in Authorization header
- Tested: register -> create habit -> list habits works correctly

internal/infrastructure/http/auth_middleware.go

@@ -0,0 +1,46 @@
+package http
+
+import (
+	"context"
+	"net/http"
+	"strings"
+
+	"habit-tracker-api/internal/infrastructure/auth"
+)
+
+type contextKey string
+
+const UserIDKey contextKey = "userID"
+
+func AuthMiddleware(jwtService *auth.JWTService) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			authHeader := r.Header.Get("Authorization")
+			if authHeader == "" {
+				respondError(w, http.StatusUnauthorized, "Missing authorization header")
+				return
+			}
+
+			parts := strings.Split(authHeader, " ")
+			if len(parts) != 2 || parts[0] != "Bearer" {
+				respondError(w, http.StatusUnauthorized, "Invalid authorization header format")
+				return
+			}
+
+			tokenString := parts[1]
+			claims, err := jwtService.ValidateToken(tokenString)
+			if err != nil {
+				respondError(w, http.StatusUnauthorized, "Invalid or expired token")
+				return
+			}
+
+			ctx := context.WithValue(r.Context(), UserIDKey, claims.UserID)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
+	}
+}
+
+func GetUserIDFromContext(ctx context.Context) (string, bool) {
+	userID, ok := ctx.Value(UserIDKey).(string)
+	return userID, ok
+}