Add RSS subscribe links with icon across blog pages, footer, and head

- Add <.rss_icon> inline SVG component in core_components.ex
- Inject RSS <link> tags into <head> on blog pages for auto-discovery
- Show RSS icon next to blog title on index and tag pages
- Show RSS icon on post pages next to back-link
- Add Subscribe section in footer with links to both feeds
- Wire feed URL through blog controller (index, show, tag actions)

174 tests pass, credo clean.

app/lib/firehose_web/controllers/blog_html/show.html.heex

@@ -1,5 +1,16 @@
 <div class="space-y-8">
-  <a href={@base_path} class="text-sm text-primary hover:underline">&larr; Back to posts</a>
+  <div class="flex items-center justify-between">
+    <a href={@base_path} class="text-sm text-primary hover:underline">&larr; Back to posts</a>
+    <a
+      href={@rss_feed_url}
+      class="link link-hover text-orange-500 hover:text-orange-600 transition-colors"
+      title="Subscribe to RSS feed"
+      target="_blank"
+      rel="noopener noreferrer"
+    >
+      <.rss_icon class="size-5" />
+    </a>
+  </div>
 
   <%= if @authenticated and @visibility == :draft do %>
     <div