add authentication method

docs/DATABASE.md

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+# Database Persistence
+
+## Overview
+
+AtlasOS now supports SQLite-based database persistence for configuration and state management. The database layer is optional - if no database path is provided, the system operates in in-memory mode (data is lost on restart).
+
+## Configuration
+
+Set the `ATLAS_DB_PATH` environment variable to enable database persistence:
+
+```bash
+export ATLAS_DB_PATH=/var/lib/atlas/atlas.db
+./atlas-api
+```
+
+If not set, the system defaults to `data/atlas.db` in the current directory.
+
+## Database Schema
+
+The database includes tables for:
+
+- **users** - User accounts and authentication
+- **audit_logs** - Audit trail with indexes for efficient querying
+- **smb_shares** - SMB/CIFS share configurations
+- **nfs_exports** - NFS export configurations
+- **iscsi_targets** - iSCSI target configurations
+- **iscsi_luns** - iSCSI LUN mappings
+- **snapshot_policies** - Automated snapshot policies
+
+## Current Status
+
+✅ **Database Infrastructure**: Complete
+- SQLite database connection and migration system
+- Schema definitions for all entities
+- Optional database mode (falls back to in-memory if not configured)
+
+⏳ **Store Migration**: In Progress
+- Stores currently use in-memory implementations
+- Database-backed implementations can be added incrementally
+- Pattern established for migration
+
+## Migration Pattern
+
+To migrate a store to use the database:
+
+1. Add database field to store struct
+2. Update `New*Store()` to accept `*db.DB` parameter
+3. Implement database queries in CRUD methods
+4. Update `app.go` to pass database to store constructor
+
+Example pattern:
+
+```go
+type UserStore struct {
+    db   *db.DB
+    mu   sync.RWMutex
+    // ... other fields
+}
+
+func NewUserStore(db *db.DB, auth *Service) *UserStore {
+    // Initialize with database
+}
+
+func (s *UserStore) Create(...) (*User, error) {
+    // Use database instead of in-memory map
+    _, err := s.db.Exec("INSERT INTO users ...")
+    // ...
+}
+```
+
+## Benefits
+
+- **Persistence**: Configuration survives restarts
+- **Audit Trail**: Historical audit logs preserved
+- **Scalability**: Can migrate to PostgreSQL/MySQL later
+- **Backup**: Simple file-based backup (SQLite database file)
+
+## Next Steps
+
+1. Migrate user store to database (highest priority for security)
+2. Migrate audit log store (for historical tracking)
+3. Migrate storage service stores (SMB/NFS/iSCSI)
+4. Migrate snapshot policy store
+5. Add database backup/restore utilities