fix(links): absolute URL for openapi.json in conventions (#238)

* fix(seo): enable trailingSlash for consistent canonical/hreflang/sitemap URLs

Semrush flagged 9 hreflang conflicts because Vercel serves both /en and
/en/ as 200 (no redirect), so whichever form tags use conflicts with the
alternate crawled URL.

Fix: enable trailingSlash: true in Next.js config so Vercel redirects
/en → /en/ (308). Update page.tsx and sitemap generator to produce
trailing-slash URLs consistently across:
- <link rel="canonical">
- <link rel="alternate" hreflang>
- sitemap.xml <loc> and <xhtml:link>

Build tested locally — static export now generates directory-based
index.html files (en/index.html instead of en.html).

* fix(ci): update check-links entry path for trailingSlash mode

With trailingSlash: true, static export produces en/index.html instead
of en.html. Update the link checker entry path from /en.html to /en/
so the CI check-links step finds the homepage.

* fix(links): absolute URL for openapi.json link in conventions

Same class of bug as asyncapi.yaml — relative /openapi.json gets
locale-prefixed to /en/openapi.json (404). Use absolute URL instead.

---------

Co-authored-by: paperclip-resolver[bot] <3736210+paperclip-resolver[bot]@users.noreply.github.com>

Author: Mlaz-code

content/de/api-reference/conventions.mdx

@@ -191,7 +191,7 @@ ETag: "9dc023776c4b382"
 
 ## Maschinenlesbare Quelle der Wahrheit
 
-- REST-Oberfläche: [`openapi.json`](/openapi.json) (OpenAPI 3.1)
+- REST-Oberfläche: [`openapi.json`](https://docs.sharpapi.io/openapi.json) (OpenAPI 3.1)
 - WebSocket-Oberfläche: [`asyncapi.yaml`](https://docs.sharpapi.io/asyncapi.yaml) (AsyncAPI 3.0)
 
 Beide Dateien werden als statische Assets veröffentlicht und können in CI verglichen werden. Wenn diese Seite von der Spezifikation abweicht, gewinnt die Spezifikation — die Spezifikation wird vom bereitgestellten Server generiert.

content/en/api-reference/conventions.mdx

@@ -191,7 +191,7 @@ ETag: "9dc023776c4b382"
 
 ## Machine-readable source of truth
 
-- REST surface: [`openapi.json`](/openapi.json) (OpenAPI 3.1)
+- REST surface: [`openapi.json`](https://docs.sharpapi.io/openapi.json) (OpenAPI 3.1)
 - WebSocket surface: [`asyncapi.yaml`](https://docs.sharpapi.io/asyncapi.yaml) (AsyncAPI 3.0)
 
 Both files are published as static assets and can be diffed in CI. If this page drifts from the spec, the spec wins — the spec is generated from the deployed server.

content/es/api-reference/conventions.mdx

@@ -191,7 +191,7 @@ ETag: "9dc023776c4b382"
 
 ## Fuente de verdad legible por máquina
 
-- Superficie REST: [`openapi.json`](/openapi.json) (OpenAPI 3.1)
+- Superficie REST: [`openapi.json`](https://docs.sharpapi.io/openapi.json) (OpenAPI 3.1)
 - Superficie WebSocket: [`asyncapi.yaml`](https://docs.sharpapi.io/asyncapi.yaml) (AsyncAPI 3.0)
 
 Ambos archivos se publican como recursos estáticos y se pueden comparar en CI. Si esta página se desvía de la especificación, la especificación gana: la especificación se genera a partir del servidor desplegado.

content/pt-BR/api-reference/conventions.mdx

@@ -191,7 +191,7 @@ ETag: "9dc023776c4b382"
 
 ## Fonte da verdade legível por máquina
 
-- Superfície REST: [`openapi.json`](/openapi.json) (OpenAPI 3.1)
+- Superfície REST: [`openapi.json`](https://docs.sharpapi.io/openapi.json) (OpenAPI 3.1)
 - Superfície WebSocket: [`asyncapi.yaml`](https://docs.sharpapi.io/asyncapi.yaml) (AsyncAPI 3.0)
 
 Ambos os arquivos são publicados como assets estáticos e podem ser comparados (diff) em CI. Se esta página divergir da especificação, a especificação prevalece — a especificação é gerada a partir do servidor implantado.