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/services/contractor_service.go

@@ -70,16 +70,12 @@ func (s *ContractorService) hasContractorAccess(contractor *models.Contractor, u
 
 // ListContractors lists all contractors accessible to a user
 func (s *ContractorService) ListContractors(userID uint) ([]responses.ContractorResponse, error) {
-	residences, err := s.residenceRepo.FindByUser(userID)
+	// Get residence IDs (lightweight - no preloads)
+	residenceIDs, err := s.residenceRepo.FindResidenceIDsByUser(userID)
 	if err != nil {
 		return nil, err
 	}
 
-	residenceIDs := make([]uint, len(residences))
-	for i, r := range residences {
-		residenceIDs[i] = r.ID
-	}
-
 	// FindByUser now handles both personal and residence contractors
 	contractors, err := s.contractorRepo.FindByUser(userID, residenceIDs)
 	if err != nil {