Graphviz diagrams set Helvetica on every node and schema label, in two
hardcoded places, so a caller had no way to ask for another typeface. A
new setting, display.diagram_fontname (environment variable
DJ_DIAGRAM_FONTNAME), names the face for both. It defaults to Helvetica,
so diagrams are unchanged for anyone who does not set it. An empty name
is refused, since Graphviz would fall back to Times.

The issue proposed a key in the diagram themes; this is a display setting
beside diagram_direction and diagram_theme instead, so the themes still
change colors only. Graphviz sizes node boxes with the face it finds on
the machine doing the rendering, and a face that machine lacks changes
the text metrics and so the layout. The setting is for callers who
control that environment. For a published figure, restyling the emitted
SVG with a CSS font stack remains the safer route.

Mermaid output is unchanged. Mermaid has a font option of its own, but
make_mermaid() leaves the typeface to the Mermaid host, as it already
leaves dark mode, so neither display.diagram_fontname nor
display.diagram_theme applies to it. Its docstring listed the Helvetica
font among the Graphviz features with no Mermaid equivalent; it now says
the typeface is left to the host.

The tests check, on MySQL and PostgreSQL, that every fontname in the dot
source is Helvetica by default, and that with the setting each is the
configured face, in the same number of places, the SVG names that face,
and the Mermaid output does not change. Both fail without the change.

The docs' guidance that diagram themes do not change fonts stays true; it
should also name this setting, with the layout caveat.

Closes #1539.