feat: user relay management, blog naddr sync, and OG image update

Add UserRelay API and dashboard relays tab with NIP-65 import, store post
naddr for Nostr articles, and ship Prisma migrations for board tables,
user relays, and post metadata.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/prisma/schema.prisma

@@ -61,9 +61,10 @@ model Media {
 model Post {
   id            String         @id @default(uuid())
   nostrEventId  String         @unique
+  naddr         String?
   title         String
   slug          String         @unique
-  content       String
+  content       String         @default("")
   excerpt       String?
   authorPubkey  String
   authorName    String?
@@ -117,6 +118,17 @@ model Relay {
   createdAt DateTime @default(now())
 }
 
+model UserRelay {
+  id        String   @id @default(uuid())
+  pubkey    String
+  url       String
+  read      Boolean  @default(true)
+  write     Boolean  @default(true)
+  createdAt DateTime @default(now())
+
+  @@unique([pubkey, url])
+}
+
 model Setting {
   id        String   @id @default(uuid())
   key       String   @unique