feat: add fediverse_creator handle for Mastodon attribution

Expose a per-post fediverse_creator frontmatter field plus a site-wide
default in config.toml ([site].fediverse_creator). The handle is returned
in the post and site payloads, rendered as <dc:creator> in the RSS feed
and as authors[].name in the JSON feed, so a front-end can drop it into
<meta name="fediverse:creator"> on rendered pages. Per-post value
overrides the site default. Validated against a simple @user@host regex.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

docs/configuration.md

@@ -22,6 +22,11 @@ description = "A blog powered by volumen."
 base_url = "https://example.com"
 language = "en"
 author = "Anonymous"
+# Site-wide fediverse handle (e.g. Mastodon). Exposed as `fediverse_creator`
+# on /api/volumen/site and used as the fallback in the RSS / JSON feeds when
+# a post does not set its own `fediverse_creator` frontmatter field. Leave
+# empty if you do not publish to the fediverse.
+fediverse_creator = "@you@example.social"
 
 [admin]
 # scrypt hash produced by `volumen hash-password`.