feat(erd): table and column comments from the database as notes #1279

Part of #1286.

Problem

Comments written in the database (COMMENT ON TABLE/COLUMN in PostgreSQL, table_comment / column_comment in MySQL) are the documentation of a schema, and the diagram ignores them. dbdiagram shows notes on tables and columns.

Scope

• Read comments in the catalog (PostgreSQL pg_description, MySQL information_schema; SQLite has none).
• A note marker on the header / column row; the text in a tooltip; the table note under the header when the card is selected.
• Long notes are wrapped and capped in the tooltip.

Acceptance

• PostgreSQL and MySQL: table and column comments appear as notes.
• No marker without a comment; SQLite unaffected.
• Catalog tests per dialect.