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/apperrors/handler.go

@@ -0,0 +1,66 @@
+package apperrors
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+
+	"github.com/go-playground/validator/v10"
+	"github.com/labstack/echo/v4"
+	"github.com/rs/zerolog/log"
+
+	"github.com/treytartt/casera-api/internal/dto/responses"
+	"github.com/treytartt/casera-api/internal/i18n"
+	customvalidator "github.com/treytartt/casera-api/internal/validator"
+)
+
+// HTTPErrorHandler handles all errors returned from handlers in a consistent way.
+// It converts AppErrors, validation errors, and Echo HTTPErrors to JSON responses.
+// This is the base handler - additional service-level error handling can be added in router.go.
+func HTTPErrorHandler(err error, c echo.Context) {
+	// Already committed? Skip
+	if c.Response().Committed {
+		return
+	}
+
+	// Handle AppError (our custom application errors)
+	var appErr *AppError
+	if errors.As(err, &appErr) {
+		message := i18n.LocalizedMessage(c, appErr.MessageKey)
+		// If i18n key not found (returns the key itself), use fallback message
+		if message == appErr.MessageKey && appErr.Message != "" {
+			message = appErr.Message
+		} else if message == appErr.MessageKey {
+			message = appErr.MessageKey // Use the key as last resort
+		}
+
+		// Log internal errors
+		if appErr.Err != nil {
+			log.Error().Err(appErr.Err).Str("message_key", appErr.MessageKey).Msg("Application error")
+		}
+
+		c.JSON(appErr.Code, responses.ErrorResponse{Error: message})
+		return
+	}
+
+	// Handle validation errors from go-playground/validator
+	var validationErrs validator.ValidationErrors
+	if errors.As(err, &validationErrs) {
+		c.JSON(http.StatusBadRequest, customvalidator.FormatValidationErrors(err))
+		return
+	}
+
+	// Handle Echo's built-in HTTPError
+	var httpErr *echo.HTTPError
+	if errors.As(err, &httpErr) {
+		msg := fmt.Sprintf("%v", httpErr.Message)
+		c.JSON(httpErr.Code, responses.ErrorResponse{Error: msg})
+		return
+	}
+
+	// Default: Internal server error (don't expose error details to client)
+	log.Error().Err(err).Msg("Unhandled error")
+	c.JSON(http.StatusInternalServerError, responses.ErrorResponse{
+		Error: i18n.LocalizedMessage(c, "error.internal"),
+	})
+}