feat: add ERD endpoint for entity relationship diagrams

Add GET /api/erd endpoint that returns table schemas with column details
(name, type, nullable, primary key) and foreign key relationships for
rendering ERD diagrams in the UI.

Implements erd() for all 7 database drivers:
- SQLite/LibSQL: PRAGMA table_info and foreign_key_list
- PostgreSQL: information_schema with table_constraints joins
- MySQL: information_schema with column_key for PKs
- DuckDB: information_schema with duckdb_constraints()
- ClickHouse: system.columns (no FK support)
- MSSQL: sys.columns/types/indexes with sys.foreign_keys

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: frectonz