feat: eliminate URL dualism — computed permalinks, conversations support

- Add computed permalink in data cascade for existing posts without
  frontmatter permalink (converts file path to Indiekit URL)
- Fix ogSlug filter and computed data for new 5-segment URL structure
- Add conversations API as build-time data source
- Merge conversations + webmentions in webmentionsForUrl filter with
  deduplication and legacy /content/ URL alias computation
- Sidebar widget fetches from both webmention-io and conversations APIs
- Update webmention-debug page with conversationMentions parameter

_data/conversationMentions.js

@@ -0,0 +1,14 @@
+import EleventyFetch from "@11ty/eleventy-fetch";
+
+export default async function () {
+  try {
+    const data = await EleventyFetch(
+      "http://127.0.0.1:8080/conversations/api/mentions?per-page=10000",
+      { duration: "15m", type: "json" }
+    );
+    return data.children || [];
+  } catch (e) {
+    console.log(`[conversationMentions] API unavailable: ${e.message}`);
+    return [];
+  }
+}