feat: implement unified S3-compatible storage system

Consolidates storage backends into a single S3-compatible driver that supports:
- AWS S3 (native)
- Backblaze B2 (S3-compatible API)
- Cloudflare R2 (S3-compatible API)
- MinIO and other S3-compatible services
- Local filesystem for development

This replaces the previous separate B2 driver with a unified approach,
reducing dependencies and complexity while adding support for more services.

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

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

database/database.go

@@ -102,6 +102,7 @@ func AutoMigrate() error {
 		&models.APIKey{},
 		&models.Invite{},
 		&models.BudgetItem{},
+		&models.Attachment{},
 	)
 }
 
@@ -131,6 +132,10 @@ func createIndexes() error {
 		"CREATE INDEX IF NOT EXISTS split_date_index ON splits(date)",
 		"CREATE INDEX IF NOT EXISTS split_updated_index ON splits(updated)",
 		"CREATE INDEX IF NOT EXISTS budgetitem_orgId_index ON budget_items(orgId)",
+		"CREATE INDEX IF NOT EXISTS attachment_transactionId_index ON attachment(transactionId)",
+		"CREATE INDEX IF NOT EXISTS attachment_orgId_index ON attachment(orgId)",
+		"CREATE INDEX IF NOT EXISTS attachment_userId_index ON attachment(userId)",
+		"CREATE INDEX IF NOT EXISTS attachment_uploaded_index ON attachment(uploaded)",
 		
 		// Additional useful indexes for performance
 		"CREATE INDEX IF NOT EXISTS idx_transaction_date ON transactions(date)",