feat: add first_or_fail to QueryBuilder and Model

[tmgbedu]

Summary

• Adds QueryBuilder.first_or_fail(columns=None) — raises ModelNotFoundException when first() returns None
• Adds Model.first_or_fail(columns=None) — class-level shortcut delegating to the builder
• Adds 4 test cases covering: found record, no match, 404 status code, and empty table

Usage

# raises ModelNotFoundException if no match
user = await User.where("email", email).first_or_fail()

# or via the builder
user = await User.query().where("is_admin", True).first_or_fail()

Test plan

• test_first_or_fail_returns_model_when_found
• test_first_or_fail_raises_when_no_match
• test_first_or_fail_exception_has_404_status
• test_first_or_fail_class_method_raises_when_table_empty

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