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/009_remove_redundant_indexes.down.sql

@@ -0,0 +1,10 @@
+-- Rollback: Recreate removed indexes
+
+-- Recreate share code index (from migration 006)
+CREATE INDEX IF NOT EXISTS idx_share_code_active
+    ON residence_residencesharecode (residence_id)
+    WHERE is_active = true;
+
+-- Recreate notification user+sent index (from migration 006)
+CREATE INDEX IF NOT EXISTS idx_notification_user_sent
+    ON notifications_notification (user_id, sent);