Implement correct relationship cardinality detection (1:1, optional/required)

[JoseVelazcoH]

Problem

All relationships are currently rendered as 1:N regardless of the actual cardinality. Two common cases are misrepresented:

1:1 relationships

When a foreign key column is also part of the primary key, the relationship is 1:1, not 1:N.

```python
class UserProfile(Base):
tablename = "user_profile"
user_id = Column(Integer, ForeignKey("user.id"), primary_key=True) # 1:1, not 1:N
```

Optional vs required (nullable FKs)

A nullable FK means the relationship is optional (0..1 or 0..N). A non-nullable FK means it is required (1 or 1..N). This distinction is currently lost.

```python
assignee_id = Column(Integer, ForeignKey("user.id"), nullable=True) # 0..N — optional
department_id = Column(Integer, ForeignKey("dept.id"), nullable=False) # 1..N — required
```

Proposed cardinality rules

FK condition	Left side	Right side
FK is also PK	1	1
FK is nullable	0..1 or 0..N	—
FK is non-nullable	1	—
FK + unique constraint	1	1

Impact

Incorrect cardinality makes the ERD misleading for schema review and documentation purposes.