Add performance optimizations and database indexes

Database Indexes (migrations 006-009):
- Add case-insensitive indexes for auth lookups (email, username)
- Add composite indexes for task kanban queries
- Add indexes for notification, document, and completion queries
- Add unique index for active share codes
- Remove redundant idx_share_code_active and idx_notification_user_sent

Repository Optimizations:
- Add FindResidenceIDsByUser() lightweight method (IDs only, no preloads)
- Optimize GetResidenceUsers() with single UNION query (was 2 queries)
- Optimize kanban completion preloads to minimal columns (id, task_id, completed_at)

Service Optimizations:
- Remove Category/Priority/Frequency preloads from task queries
- Remove summary calculations from CRUD responses (client calculates)
- Use lightweight FindResidenceIDsByUser() instead of full FindByUser()

These changes reduce database load and response times for common operations.

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

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

internal/dto/responses/task.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/treytartt/casera-api/internal/models"
 	"github.com/treytartt/casera-api/internal/task/categorization"
+	"github.com/treytartt/casera-api/internal/task/predicates"
 )
 
 // TaskCategoryResponse represents a task category
@@ -234,7 +235,7 @@ func NewTaskResponseWithThreshold(t *models.Task, daysThreshold int) TaskRespons
 		IsCancelled:     t.IsCancelled,
 		IsArchived:      t.IsArchived,
 		ParentTaskID:    t.ParentTaskID,
-		CompletionCount: len(t.Completions),
+		CompletionCount: predicates.GetCompletionCount(t),
 		KanbanColumn:    DetermineKanbanColumn(t, daysThreshold),
 		CreatedAt:       t.CreatedAt,
 		UpdatedAt:       t.UpdatedAt,