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/api-reference.md

@@ -16,10 +16,14 @@ Returns site metadata from the configuration.
   "description": "A blog powered by volumen.",
   "base_url": "https://example.com",
   "language": "en",
-  "author": "Anonymous"
+  "author": "Anonymous",
+  "fediverse_creator": "@petrbalvin@mastodon.social"
 }
 ```
 
+`fediverse_creator` is `null` when the site-wide default is not set; a per-post
+value (see below) takes precedence when present.
+
 ## `GET /api/volumen/posts`
 
 Paginated list of published posts (drafts are excluded), newest first.
@@ -46,6 +50,7 @@ Posts with `all_langs = true` in their frontmatter match every `lang` filter.
       "lang": "en",
       "tags": ["intro"],
       "author": null,
+      "fediverse_creator": null,
       "cover": null,
       "reading_time": 1,
       "translations": {},
@@ -70,6 +75,7 @@ no match.
   "lang": "en",
   "tags": ["intro"],
   "author": null,
+  "fediverse_creator": "@petrbalvin@mastodon.social",
   "cover": null,
   "reading_time": 1,
   "translations": {},

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+# empty if you do not publish to the fediverse.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`.