New store methods on internal/db/ports.go:
- CreatePort / GetPort / UpdatePort / DeletePort (all project-scoped)
- ListPortsForDevice for the inspector's per-device list
New handlers (internal/server/ports.go):
- GET /api/projects/:pid/devices/:id/ports
- POST /api/projects/:pid/devices/:id/ports ← {type_id, label?, x_offset, y_offset}
- PATCH /api/projects/:pid/ports/:id ← partial
- DELETE /api/projects/:pid/ports/:id (cables ref → ON DELETE SET NULL)
Lets slice 7's +Port tool add/remove instance ports without going
through the type-seeded auto-creation path from slice 4.