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>

migrations/008_additional_performance_indexes.down.sql

@@ -0,0 +1,8 @@
+-- Rollback additional performance optimization indexes
+-- Migration: 008_additional_performance_indexes
+
+DROP INDEX IF EXISTS idx_task_kanban_composite;
+DROP INDEX IF EXISTS idx_completion_task_date;
+DROP INDEX IF EXISTS idx_sharecode_code_active;
+DROP INDEX IF EXISTS idx_residence_users_user_residence;
+DROP INDEX IF EXISTS idx_task_in_progress;