feat: Phase 1 - Enhanced feed discovery with validation

- Add validator.js: validateFeedUrl with comments feed detection
- Add discovery.js: discoverAndValidateFeeds with type labels
- Add opml.js: OPML 2.0 export of all subscriptions
- Update reader.js: searchFeeds uses validation, subscribe validates
- Update feeds.js: updateFeedStatus for health tracking
- Update search.njk: Show feed types, validation status, error messages
- Add CSS for badges, notices, and invalid feed styling
- Register OPML export route at /reader/opml

Phase 1 of blogroll implementation plan.

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

index.js

@@ -3,6 +3,7 @@ import path from "node:path";
 import express from "express";
 
 import { microsubController } from "./lib/controllers/microsub.js";
+import { opmlController } from "./lib/controllers/opml.js";
 import { readerController } from "./lib/controllers/reader.js";
 import { handleMediaProxy } from "./lib/media/proxy.js";
 import { startScheduler, stopScheduler } from "./lib/polling/scheduler.js";
@@ -97,6 +98,7 @@ export default class MicrosubEndpoint {
     readerRouter.post("/search", readerController.searchFeeds);
     readerRouter.post("/subscribe", readerController.subscribe);
     readerRouter.post("/api/mark-read", readerController.markAllRead);
+    readerRouter.get("/opml", opmlController.exportOpml);
     router.use("/reader", readerRouter);
 
     return router;