feat: Implement database metadata API

[ApostolisFalaras]

Description

Expose database catalog metadata required by the frontend schema explorer.

Tasks

• Add GET /api/schema
• Return available tables
• Return table columns and data types
• Return table constraints
• Return primary indexes
• Return secondary indexes
• Return index key columns
• Return row counts when available
• Handle empty databases
• Propagate catalog errors safely

Acceptance Criteria

• The frontend can populate its database explorer dynamically.
• Table metadata is returned in a stable JSON format.
• Index metadata is included.
• Empty databases return valid empty collections.

Depends On

• feat: Implement DBMS request-response adapter
• feat: Implement persisted system catalog