feat(matrix): render bot messages with HTML formatting

Add HTML formatted_body to Matrix bot messages so that markdown-style
formatting (code blocks, bold, italic, lists) renders properly in Matrix
clients. Uses the pulldown-cmark crate to convert markdown to HTML and
sets the message format to org.matrix.custom.html.

Story: 188_story_render_matrix_bot_messages_with_html_formatting

server/Cargo.toml

@@ -29,6 +29,7 @@ toml = { workspace = true }
 uuid = { workspace = true, features = ["v4", "serde"] }
 walkdir = { workspace = true }
 matrix-sdk = { workspace = true }
+pulldown-cmark = { workspace = true }
 
 [dev-dependencies]
 tempfile = { workspace = true }