V101: Projects module tables.

Creates five tables used by phoenix_kit_projects:

• phoenix_kit_project_tasks — reusable task library with default assignees and estimated duration
• phoenix_kit_project_task_dependencies — "task A must finish before task B" links at the template (library) level
• phoenix_kit_projects — project containers with start mode (immediate / scheduled)
• phoenix_kit_project_assignments — task instance in a project; copies duration + description from template (editable independently)
• phoenix_kit_project_dependencies — per-project assignment-level "A must finish before B" links

Each assignment and task template may have at most one assignee (team, department, or person) — enforced by CHECK (num_nonnulls(...) <= 1) on both tables.

Depends on V100 (staff tables) for the polymorphic assignee foreign keys.