feat: add feed auto-discovery to blog add form

- Add feed-discovery.js utility that discovers RSS/Atom/JSON feeds from website URLs
- Add /api/discover endpoint for frontend feed discovery
- Update blog edit form with discovery UI (enter website URL, discover feeds)
- Auto-populate feedUrl, title, and siteUrl from discovery results
- Handle multiple feed options (let user choose)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

locales/en.json

@@ -91,6 +91,17 @@
       "deleted": "Blog deleted successfully.",
       "refreshed": "Blog refreshed. Added %{items} new items.",
       "form": {
+        "discoverUrl": "Website URL",
+        "discover": "Discover Feed",
+        "discoverHint": "Enter a website URL to auto-discover its RSS/Atom feed",
+        "discoverNoUrl": "Please enter a website URL",
+        "discovering": "Discovering...",
+        "discoveringHint": "Checking for RSS/Atom feeds...",
+        "discoverFailed": "Failed to discover feeds",
+        "discoverNoFeeds": "No feeds found on this website",
+        "discoverFoundOne": "Found feed:",
+        "discoverFoundMultiple": "Multiple feeds found. Click one to select:",
+        "discoverSelected": "Selected feed:",
         "feedUrl": "Feed URL",
         "feedUrlHint": "RSS, Atom, or JSON Feed URL",
         "title": "Title",