Fix compile-time link validation and add tag page links

- Restore HTML link extraction in LinkValidator (removed in a83634d
  under the false premise that post bodies are raw markdown; they
  are HTML rendered by NimblePublisher at compile time). The missing
  regex made extract_links/1 find zero links, silently disabling
  compile-time validation.
- Support /blog/{blog_id}/tag/{tag} links: validate blog ID,
  require non-empty tag (tags are user-defined, e.g. pi.dev).
- Fix invalid links in two posts: tag/Pi.dev -> tag/pi.dev,
  2026-07-13-synthetic-tdd.md -> synthetic-tdd.
- Fix test warnings: use Plug.Test deprecation, unused import,
  runtime-defined TestBlogValid module.
- Add regression tests for HTML extraction and tag page links.

blogex/test/blogex/router_test.exs

@@ -1,6 +1,6 @@
 defmodule Blogex.RouterTest do
   use ExUnit.Case
-  use Plug.Test
+  import Plug.Test
 
   import Blogex.Test.PostBuilder
   alias Blogex.Test.FakeBlog