feat(erd): nullability, optional/mandatory edge ends, edge tooltip #1156

Problem

• Nullability is not loaded by ErdCatalog, so the crow's foot always means "many": "zero or many" (nullable FK) and "one or many" look the same, and a nullable column is not marked on the card.
• An edge has no label on screen; which columns it joins is visible only by following it to the rows (the SVG has a <title>, the screen does not).

Scope

• Catalog: is_nullable for PostgreSQL / MySQL (information_schema.columns), pragma_table_info.notnull for SQLite; ErdColumn.isNullable.
• Card: nullable columns marked (muted ? or null after the type).
• Edge ends: optional side drawn with a circle (crow's foot + circle for a nullable FK), mandatory with a bar; the same in SVG.
• Hover on an edge: tooltip orders.customer_id → customers.id (hit test on the route with a few px tolerance).

Acceptance

• Nullable FK and NOT NULL FK draw different end markers, on screen and in SVG.
• Hovering an edge shows its columns.
• Catalog tests per dialect include nullability.