Migrate from Gin to Echo framework and add comprehensive integration tests

Major changes:
- Migrate all handlers from Gin to Echo framework
- Add new apperrors, echohelpers, and validator packages
- Update middleware for Echo compatibility
- Add ArchivedHandler to task categorization chain (archived tasks go to cancelled_tasks column)
- Add 6 new integration tests:
  - RecurringTaskLifecycle: NextDueDate advancement for weekly/monthly tasks
  - MultiUserSharing: Complex sharing with user removal
  - TaskStateTransitions: All state transitions and kanban column changes
  - DateBoundaryEdgeCases: Threshold boundary testing
  - CascadeOperations: Residence deletion cascade effects
  - MultiUserOperations: Shared residence collaboration
- Add single-purpose repository functions for kanban columns (GetOverdueTasks, GetDueSoonTasks, etc.)
- Fix RemoveUser route param mismatch (userId -> user_id)
- Fix determineExpectedColumn helper to correctly prioritize in_progress over overdue

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/validator/validator.go

@@ -0,0 +1,102 @@
+package validator
+
+import (
+	"net/http"
+	"reflect"
+	"strings"
+
+	"github.com/go-playground/validator/v10"
+	"github.com/labstack/echo/v4"
+)
+
+// CustomValidator wraps go-playground/validator for Echo
+type CustomValidator struct {
+	validator *validator.Validate
+}
+
+// NewCustomValidator creates a new validator instance
+func NewCustomValidator() *CustomValidator {
+	v := validator.New()
+
+	// Use JSON tag names for field names in errors
+	v.RegisterTagNameFunc(func(fld reflect.StructField) string {
+		name := strings.SplitN(fld.Tag.Get("json"), ",", 2)[0]
+		if name == "-" {
+			return ""
+		}
+		return name
+	})
+
+	return &CustomValidator{validator: v}
+}
+
+// Validate implements echo.Validator interface
+func (cv *CustomValidator) Validate(i interface{}) error {
+	if err := cv.validator.Struct(i); err != nil {
+		return err
+	}
+	return nil
+}
+
+// ValidationErrorResponse is the structured error response format
+type ValidationErrorResponse struct {
+	Error  string                `json:"error"`
+	Fields map[string]FieldError `json:"fields,omitempty"`
+}
+
+// FieldError represents a single field validation error
+type FieldError struct {
+	Message string `json:"message"`
+	Tag     string `json:"tag"`
+}
+
+// FormatValidationErrors converts validator errors to structured response
+func FormatValidationErrors(err error) *ValidationErrorResponse {
+	if validationErrors, ok := err.(validator.ValidationErrors); ok {
+		fields := make(map[string]FieldError)
+		for _, fe := range validationErrors {
+			fieldName := fe.Field()
+			fields[fieldName] = FieldError{
+				Message: formatMessage(fe),
+				Tag:     fe.Tag(),
+			}
+		}
+		return &ValidationErrorResponse{
+			Error:  "Validation failed",
+			Fields: fields,
+		}
+	}
+	return &ValidationErrorResponse{Error: err.Error()}
+}
+
+// HTTPError returns an echo.HTTPError with validation details
+func HTTPError(c echo.Context, err error) error {
+	return c.JSON(http.StatusBadRequest, FormatValidationErrors(err))
+}
+
+func formatMessage(fe validator.FieldError) string {
+	switch fe.Tag() {
+	case "required":
+		return "This field is required"
+	case "required_without":
+		return "This field is required when " + fe.Param() + " is not provided"
+	case "required_with":
+		return "This field is required when " + fe.Param() + " is provided"
+	case "email":
+		return "Must be a valid email address"
+	case "min":
+		return "Must be at least " + fe.Param() + " characters"
+	case "max":
+		return "Must be at most " + fe.Param() + " characters"
+	case "len":
+		return "Must be exactly " + fe.Param() + " characters"
+	case "oneof":
+		return "Must be one of: " + fe.Param()
+	case "url":
+		return "Must be a valid URL"
+	case "uuid":
+		return "Must be a valid UUID"
+	default:
+		return "Invalid value"
+	}
+}