feat(core): prepare data models for multi-user support

[guglielmo]

Description

Prepare data models for future multi-user support without implementing full multi-user functionality.

Tasks

• Add optional user_id field to Resource model (default: "default_user")
• Update StorageService to filter by user context
• Create database migration framework
• Document user context pattern for future implementation

Acceptance Criteria

• Existing single-user functionality unchanged
• Data models ready for multi-user extension
• Migration path documented

Note

This prepares the foundation only. Full multi-user implementation is deferred to a future milestone.