Add comprehensive database storage layer for new features:
- db_groups.go: Lab group management with members, roles (admin/member/viewer),
and group-based task visibility queries
- db_tasks.go: Task visibility system (private/lab/institution/open),
task sharing with expiry, public clone tokens, and optimized
ListTasksForUser() for access control
- db_tokens.go: Secure token management for public task access and cloning,
with SHA-256 hashed token storage and automatic cleanup
- db_audit.go: Audit log persistence with checkpoint chains, tamper
detection, and log rotation support
- schema_sqlite.sql: Updated schema with:
- groups, group_members tables
- tasks.visibility enum, task_shares with expiry
- access_tokens table with hashed tokens
- audit_logs, audit_checkpoints tables
- indexes for all foreign keys and query patterns
- db_experiments.go: Add CascadeVisibilityToTasks() for propagating
visibility changes from experiments to associated tasks
