Construct AST from Parsed Components

[stifskere]

Sub-issue of #3

Objective

Convert parsed SQL components into an AST representing the full schema.

Tasks

Stage 1: PostgreSQL CREATE TABLE (priority)

• Map parsed PostgreSQL CREATE TABLE data into AST nodes.
• Ensure proper parent-child relations (table → columns, constraints).
• Validate AST structure correctness.

Stage 2: Full MVP / Other Dialects

• Support additional node types from extended parsing.
• Improve validation for complex schema constructs.

Note: Stage 2 is deferred; focus on Stage 1 for use within Flaky.