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/models/task.go

@@ -91,6 +91,11 @@ type Task struct {
 
 	// Completions
 	Completions   []TaskCompletion `gorm:"foreignKey:TaskID" json:"completions,omitempty"`
+
+	// CompletionCount is a computed field populated via subquery for optimized queries.
+	// When populated (>= 0 and Completions slice is empty), predicates should use this
+	// instead of len(Completions) to avoid N+1 queries.
+	CompletionCount int `gorm:"-:all" json:"-"`
 }
 
 // TableName returns the table name for GORM
@@ -116,7 +121,9 @@ func (t *Task) IsOverdue() bool {
 		return false
 	}
 	// Completed check: NextDueDate == nil AND has completions
-	if t.NextDueDate == nil && len(t.Completions) > 0 {
+	// Supports both preloaded Completions and computed CompletionCount
+	hasCompletions := len(t.Completions) > 0 || t.CompletionCount > 0
+	if t.NextDueDate == nil && hasCompletions {
 		return false
 	}
 	// Effective date: NextDueDate ?? DueDate
@@ -143,7 +150,9 @@ func (t *Task) IsDueSoon(days int) bool {
 		return false
 	}
 	// Completed check: NextDueDate == nil AND has completions
-	if t.NextDueDate == nil && len(t.Completions) > 0 {
+	// Supports both preloaded Completions and computed CompletionCount
+	hasCompletions := len(t.Completions) > 0 || t.CompletionCount > 0
+	if t.NextDueDate == nil && hasCompletions {
 		return false
 	}
 	// Effective date: NextDueDate ?? DueDate