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/content.ts

@@ -235,6 +235,8 @@ export interface SiteContent {
   podcastFeaturedLinks: PodcastFeaturedLink[]
   seo: SeoSettings
   legal: LegalSettings
+  // ── Episodes page ──
+  episodesSeoIntro: string
 }
 
 export const DEFAULTS: SiteContent = {
@@ -411,6 +413,7 @@ export const DEFAULTS: SiteContent = {
     },
   ],
   podcastFeaturedLinks: [],
+  episodesSeoIntro: 'Verse by Verse with Nate is an expository Bible teaching podcast where we go through Scripture one verse at a time. Each episode digs into the text carefully and practically, helping you understand what the Bible says, what it means, and how to live it out. New episodes released regularly — subscribe on Spotify or your favorite podcast platform.',
   seo: {
     title: 'Verse by Verse with Nate',
     description: "Verse by Verse with Nate explores Scripture one verse at a time with practical Bible teaching.",