implement a summary property on Content

Allow the user to set a summary and make Article extract its own summary
when the Article.render is called and the summary is still missing.
Extracted summaries only work when the first item of a markdown file is
a paragraph.

omicron/ssg/content.py

@@ -11,6 +11,7 @@ if TYPE_CHECKING:
 import yaml
 from markupsafe import Markup
 from markdown_it import MarkdownIt
+from markdown_it.token import Token
 from pygments import highlight as pygments_highlight
 from pygments.formatters import HtmlFormatter
 from pygments.lexers import get_lexer_by_name
@@ -44,6 +45,9 @@ def highlight(code: str, lang: str, attrs: str) -> str:
     return pygments_highlight(code, lexer, html_formatter)
 
 
+md = MarkdownIt(options_update={"highlight": highlight})
+
+
 def read_frontmatter(path: Path) -> dict[str, Any]:
     lines = []
     with path.open("r", encoding="utf-8") as f:
@@ -88,6 +92,7 @@ class Content(ABC):
         created: date,
         updated: date | None = None,
         tags: set[str] | None = None,
+        summary: str | None = None,
     ):
         if tags is None:
             tags = set()
@@ -96,6 +101,7 @@ class Content(ABC):
         self.created = created
         self.updated = updated
         self.tags: set[str] = tags
+        self.summary: str | None = summary
         self.uri: str = self.build_uri(section, slug)
 
     @staticmethod
@@ -131,14 +137,27 @@ class Article(Content):
             meta["date"],
             meta.get("updated", None),
             meta.get("tags", None),
+            meta.get("summary", None),
         )
         self.title: str = meta["title"]
         self.content: Markup = Markup("")
 
+    @staticmethod
+    def extract_summary(tokens: list[Token]) -> str:
+        if not tokens or tokens[0].type != "paragraph_open" or tokens[0].hidden:
+            raise ContentError(
+                "Body does not start with a paragraph; set 'summary' in frontmatter"
+            )
+        for j in range(1, len(tokens)):
+            if tokens[j].type == "paragraph_close":
+                return cast(str, md.renderer.render(tokens[1:j], md.options, {}))
+        raise ContentError("Content has no paragraph to use as summary")
+
     def render(self, site: Site) -> None:
-        md = MarkdownIt()
-        md.options["highlight"] = highlight
-        self.content = Markup(md.render(self.read_body()))
+        tokens = md.parse(self.read_body())
+        if self.summary is None:
+            self.summary = Article.extract_summary(tokens)
+        self.content = Markup(md.renderer.render(tokens, md.options, {}))
 
         template = site.jinja_env.get_template("article.html")
         html = template.render(article=self, site=site)