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/middleware/timezone.go

@@ -3,7 +3,7 @@ package middleware
 import (
 	"time"
 
-	"github.com/gin-gonic/gin"
+	"github.com/labstack/echo/v4"
 )
 
 const (
@@ -22,21 +22,23 @@ const (
 // or a UTC offset (e.g., "-08:00", "+05:30").
 //
 // If no timezone is provided or it's invalid, UTC is used as the default.
-func TimezoneMiddleware() gin.HandlerFunc {
-	return func(c *gin.Context) {
-		tzName := c.GetHeader(TimezoneHeader)
-		loc := parseTimezone(tzName)
+func TimezoneMiddleware() echo.MiddlewareFunc {
+	return func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(c echo.Context) error {
+			tzName := c.Request().Header.Get(TimezoneHeader)
+			loc := parseTimezone(tzName)
 
-		// Store the location and the current time in that timezone
-		c.Set(TimezoneKey, loc)
+			// Store the location and the current time in that timezone
+			c.Set(TimezoneKey, loc)
 
-		// Calculate "now" in the user's timezone, then get start of day
-		// For date comparisons, we want to compare against the START of the user's current day
-		userNow := time.Now().In(loc)
-		startOfDay := time.Date(userNow.Year(), userNow.Month(), userNow.Day(), 0, 0, 0, 0, loc)
-		c.Set(UserNowKey, startOfDay)
+			// Calculate "now" in the user's timezone, then get start of day
+			// For date comparisons, we want to compare against the START of the user's current day
+			userNow := time.Now().In(loc)
+			startOfDay := time.Date(userNow.Year(), userNow.Month(), userNow.Day(), 0, 0, 0, 0, loc)
+			c.Set(UserNowKey, startOfDay)
 
-		c.Next()
+			return next(c)
+		}
 	}
 }
 
@@ -76,22 +78,22 @@ func parseTimezone(tz string) *time.Location {
 	return time.UTC
 }
 
-// GetUserTimezone retrieves the user's timezone from the Gin context.
+// GetUserTimezone retrieves the user's timezone from the Echo context.
 // Returns UTC if not set.
-func GetUserTimezone(c *gin.Context) *time.Location {
-	loc, exists := c.Get(TimezoneKey)
-	if !exists {
+func GetUserTimezone(c echo.Context) *time.Location {
+	loc := c.Get(TimezoneKey)
+	if loc == nil {
 		return time.UTC
 	}
 	return loc.(*time.Location)
 }
 
-// GetUserNow retrieves the timezone-aware "now" time from the Gin context.
+// GetUserNow retrieves the timezone-aware "now" time from the Echo context.
 // This represents the start of the current day in the user's timezone.
 // Returns time.Now().UTC() if not set.
-func GetUserNow(c *gin.Context) time.Time {
-	now, exists := c.Get(UserNowKey)
-	if !exists {
+func GetUserNow(c echo.Context) time.Time {
+	now := c.Get(UserNowKey)
+	if now == nil {
 		return time.Now().UTC()
 	}
 	return now.(time.Time)