feat: full article fetching with trafilatura + html2text cleanup

- Add trafilatura + html2text to dependencies
- Replace custom HTMLStripper with html2text for RSS feed content
- Fetch full article text via httpx + trafilatura after each new item is stored;
  falls back to RSS-provided content if fetch/extraction fails
- Raise CONTENT_MAX_CHARS from 2000 to 50000 (TEXT column, no migration needed)
- Re-embed items with full article content once enrichment completes
- Startup backfill enriches existing items with short content (<1000 chars)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/app.py

@@ -266,6 +266,11 @@ def create_app() -> Quart:
                 await backfill_rss_item_embeddings()
             except Exception:
                 logger.warning("RSS embedding backfill failed", exc_info=True)
+            try:
+                from fabledassistant.services.embeddings import backfill_rss_article_content
+                await backfill_rss_article_content()
+            except Exception:
+                logger.warning("RSS article content backfill failed", exc_info=True)
 
         asyncio.create_task(_delayed_backfill())