Bug: `UUID` primary key fields create `INTEGER` columns instead of `UUID`

[roselleebarle04]

Description

When a model defines an ID field as a UUID with primary_key=True, the ORM incorrectly creates a BIGSERIAL (auto-incrementing integer) column instead of a UUID column.

Steps to Reproduce

1. Define a model with a UUID primary key:

from uuid import UUID
from airmodel import AirModel, AirField

class MyModel(AirModel):
    id: UUID = AirField(primary_key=True)
    name: str

2. Call _column_defs() to inspect the generated column definitions:

cols = MyModel._column_defs()
print(cols[0])
# Expected: '"id" UUID PRIMARY KEY'
# Actual (before fix): '"id" BIGSERIAL PRIMARY KEY'

Expected Behavior

UUID primary key fields should create UUID PRIMARY KEY columns in PostgreSQL, not BIGSERIAL PRIMARY KEY columns.

Actual Behavior

UUID primary key fields are incorrectly mapped to BIGSERIAL columns, which are auto-incrementing integers.

Root Cause

The _column_defs() method in src/airmodel/main.py assumes all primary keys should be BIGSERIAL without checking the actual field type.

Environment

• OS: macOS 26.2 (Darwin 25.2.0)
• Python: 3.13.3
• AirModel: Latest main branch

Fix

The fix checks the primary key field's type and uses the appropriate PostgreSQL type:

• int types → BIGSERIAL PRIMARY KEY
• UUID types → UUID PRIMARY KEY
• Other types → appropriate type + PRIMARY KEY