Add sponsors system with time slider, LNbits invoices, and UX improvements

- Sponsors table, LNbits createInvoice, webhook handler
- Sponsor routes: create, homepage, list, my-ads, click, extend, check-payment
- Admin routes for sponsor management
- Frontend: SponsorForm, SponsorTimeSlider, SponsorCard, SponsorsSection
- Sponsors page, My Ads page, homepage sponsor block
- Header login dropdown with My Ads, Create Sponsor
- Transactions integration for sponsor payments
- View/click tracking
- OG meta fetch for sponsor images
- Sponsor modal spacing, invoice polling fallback
- Remove Lightning address and Category fields from sponsor form

Made-with: Cursor

backend/.env.example

@@ -53,3 +53,12 @@ DEPOSIT_LNURLP=https://yourdomain.com/.well-known/lnurlp/faucet
 
 # Cashu redeem (optional; default: https://cashu-redeem.azzamo.net)
 # CASHU_REDEEM_API_URL=https://cashu-redeem.azzamo.net
+
+# Sponsors
+BASE_SPONSOR_PRICE_PER_DAY=200
+SPONSOR_MAX_ACTIVE_PER_USER=5
+SPONSOR_MAX_VISIBLE=6
+# Comma-separated hex pubkeys for admin API
+# ADMIN_PUBKEYS=abc123...,def456...
+# Public API URL for LNbits webhook (e.g. https://api.example.com)
+# PUBLIC_API_URL=https://api.example.com