feat(redirects): send old Stack URLs to their tsuki URLs

Generate a Cloudflare Pages _redirects file with 301s from the old
/post/YYYY/MM/DD/ post URLs and the old pagination pages. The rules are
temporary and marked for removal once the old URLs stop getting traffic.

config/_default/hugo.yml

@@ -7,3 +7,21 @@ hasCJKLanguage: false
 enableRobotsTXT: true
 timeZone: Asia/Ho_Chi_Minh
 enableGitInfo: true
+
+# TEMPORARY (added 2026-10-04): Cloudflare Pages `_redirects` file that sends
+# the old Stack-theme URLs (/post/YYYY/MM/DD/, home pagination /page/N/) to
+# their tsuki URLs. Remove this block and layouts/home.redirects together once
+# the old URLs no longer get traffic (review at 3, 6 or 12 months after the
+# migration ships).
+mediaTypes:
+  text/redirects:
+    delimiter: ""
+    suffixes: [""]
+outputFormats:
+  redirects:
+    baseName: _redirects
+    mediaType: text/redirects
+    isPlainText: true
+    notAlternative: true
+outputs:
+  home: [html, rss, redirects]

layouts/home.redirects

@@ -0,0 +1,22 @@
+{{- /*
+  TEMPORARY (added 2026-10-04): 301 redirects from the old Stack-theme URLs to
+  the tsuki URLs, written to public/_redirects for Cloudflare Pages.
+  Delete this file and the matching block in config/_default/hugo.yml once the
+  old URLs no longer get traffic (review at 3, 6 or 12 months after the
+  migration ships).
+
+  - Posts: Stack had no post permalink rule, so a post lived at its content
+    path, /post/YYYY/MM/DD/. tsuki serves it at /p/:slug/.
+  - Pagination: Stack paginated lists with 5 entries per page. The home page
+    pages (/page/N/) go to /, since tsuki's home page is not paginated, and the
+    page section's pages (/page/page/N/) go to its first page, /page/.
+*/ -}}
+{{- $posts := where site.RegularPages "Section" "post" -}}
+{{- range $posts }}
+/{{ .File.Dir }} {{ .RelPermalink }} 301
+{{- end }}
+{{- /* Page counts of the last Stack build; the old URL set is frozen. */ -}}
+{{- range seq 38 }}
+/page/{{ . }}/ / 301
+{{- end }}
+/page/page/2/ /page/ 301