feat: homepage newsletter, Q&A callout, episodes page redesign, SEO/sitemap, asset tagging

- Fix "Invalid Date" on student account page (memberSince guard)
- Fix study community author showing full email instead of username
- Add newsletter sign-up section to homepage (between PRISM and Current Series)
- Add newsletter opt-in nudge to study enrollment flow
- Redesign episodes page: latest episode featured card + per-episode Spotify embed
- Add SEO description field to episodes page (visually hidden, admin-editable)
- Expand default sitemap paths to include /about, /contact, /episodes, /resources, /study, /study/titus, /study/colossians
- Add featured Q&A callout section to homepage
- Add Q&A callout to study hub footer
- Tag both untagged image assets in Asset Manager

src/AdminPage.tsx

@@ -491,6 +491,7 @@ const FIELDS: Array<{ key: StringField; label: string; multiline?: boolean; sect
   { key: 'contactPoint2', label: 'Contact — Point 2', section: 'contact' },
   { key: 'contactVerse', label: 'Contact — Scripture Text', section: 'contact' },
   { key: 'contactVerseRef', label: 'Contact — Scripture Reference', section: 'contact' },
+  { key: 'episodesSeoIntro', label: 'Episodes Page — SEO Description (hidden from visitors, indexed by search engines)', multiline: true, section: 'series' },
   { key: 'seriesLabel', label: 'Series Label (e.g. "Now Playing")', section: 'series' },
   { key: 'seriesTitle', label: 'Series Title', section: 'series' },
   { key: 'seriesDescription', label: 'Series Description', multiline: true, section: 'series' },