feat: Implement table metadata API

[ApostolisFalaras]

Description

Expose detailed metadata for an individual table when selected in the frontend database explorer.

Tasks

• Add GET /api/tables/:tableName
• Validate the table name
• Return column definitions
• Return constraints
• Return primary index metadata
• Return secondary index metadata
• Return table statistics when available
• Handle missing tables

Acceptance Criteria

• Selecting a table can retrieve its metadata.
• Missing tables return a not-found response.
• Column, constraint, and index metadata are represented consistently.

Depends On

• feat: Implement database metadata API