feat: render Markdown, mathematics and Mermaid diagrams server-side

internal/diagram/testdata/sq-activation.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 348 300" font-family="sans-serif"><line x1="99" y1="68" x2="99" y2="276" stroke="#999" stroke-dasharray="5 5"/><line x1="249" y1="68" x2="249" y2="276" stroke="#999" stroke-dasharray="5 5"/><rect x="94" y="102" width="10" height="132" fill="#e8e8e8" stroke="#666" stroke-width="1"/><rect x="36" y="24" width="126" height="44" rx="8" fill="#eef2f7" stroke="#333" stroke-width="1.5"/><text x="99" y="51" text-anchor="middle" font-size="14" font-weight="bold">A</text><rect x="186" y="24" width="126" height="44" rx="8" fill="#eef2f7" stroke="#333" stroke-width="1.5"/><text x="249" y="51" text-anchor="middle" font-size="14" font-weight="bold">B</text><line x1="99" y1="144" x2="249" y2="144" stroke="#333" stroke-width="1.8"/><polygon points="249,144 238,139 238,149" fill="#333"/><text x="174" y="137" text-anchor="middle" font-size="13">request</text><line x1="249" y1="210" x2="99" y2="210" stroke="#333" stroke-width="1.8" stroke-dasharray="7 5"/><polygon points="99,210 110,205 110,215" fill="#333"/><text x="174" y="203" text-anchor="middle" font-size="13">response</text></svg>