fix(backend): load DATABASE_URL for Prisma CLI from repo root .env

Use dotenv-cli for db:push, db:seed, db:studio, and migrate:deploy so they
match the API (root .env). Load dotenv in seed.ts for direct runs.
Document absolute DATABASE_URL in .env.example.

Made-with: Cursor

.env.example

@@ -4,7 +4,7 @@ ADMIN_PUBKEYS=npub1examplepubkey1,npub1examplepubkey2
 # Nostr relays (comma-separated)
 RELAYS=wss://relay.damus.io,wss://nos.lol,wss://relay.nostr.band
 
-# Database
+# Database (use an absolute `file:` URL in production, e.g. file:/home/bbe/BelgianBitcoinEmbassy/backend/prisma/prod.db)
 DATABASE_URL="file:./dev.db"
 
 # JWT