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>

README.md

@@ -1,10 +1,10 @@
 # Casera API (Go)
 
-Go implementation of the Casera property management API, built with Gin, GORM, and GoAdmin.
+Go implementation of the Casera property management API, built with Echo, GORM, and GoAdmin.
 
 ## Tech Stack
 
-- **HTTP Framework**: [Gin](https://github.com/gin-gonic/gin)
+- **HTTP Framework**: [Echo v4](https://github.com/labstack/echo)
 - **ORM**: [GORM](https://gorm.io/) with PostgreSQL
 - **Push Notifications**: Direct APNs (via [apns2](https://github.com/sideshow/apns2)) + FCM HTTP API
 - **Admin Panel**: [GoAdmin](https://github.com/GoAdminGroup/go-admin)