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>

pyproject.toml

@@ -19,6 +19,8 @@ dependencies = [
     "caldav>=1.3",
     "icalendar>=5.0",
     "feedparser>=6.0",
+    "html2text>=2024.2",
+    "trafilatura>=1.12",
     "APScheduler>=3.10,<4.0",
     "pywebpush>=2.0",
 ]