CLI: Add database migration command (taskflow migrate)

[mjunaidca]

Summary

Add a CLI command for database migrations that supports multiple database backends, particularly useful for migrating between SQLite (local development) and PostgreSQL/Neon (production).

Proposed Command

taskflow migrate --to neon
taskflow migrate --to postgres --url "postgresql://..."
taskflow migrate --to sqlite --path "./local.db"

Context

This emerged from Phase 2 backend API development. The team needs a way to:

1. Migrate local SQLite development databases to production Neon/PostgreSQL
2. Support the development → staging → production workflow
3. Export/import data between environments

Acceptance Criteria

• taskflow migrate command with --to target option
• Support SQLite, PostgreSQL, and Neon backends
• Data validation during migration
• Rollback capability on failure
• Progress reporting for large datasets
• Audit log entries for migration operations

Technical Notes

• Should leverage existing SQLModel/SQLAlchemy models
• Consider using Alembic for schema migrations
• Data migration separate from schema migration

Priority

Future enhancement - not blocking current phase work

Labels

• enhancement
• cli
• future