refactor: Prisma schema 添加显式 ProjectTag 关联表并优化索引

- 将 Project-Tag 多对多关系改为显式 ProjectTag 中间表
- 为 ExternalLink 添加 url 索引和 type+url 复合索引
- 添加 projectId+url 唯一约束防止重复链接

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

prisma/schema.prisma

@@ -46,7 +46,7 @@ model Project {
   updatedAt      DateTime      @updatedAt
 
   // Relations
-  tags           Tag[]
+  tags           ProjectTag[]
   links          ExternalLink[]
 
   // Indexes
@@ -56,14 +56,14 @@ model Project {
 }
 
 model Tag {
-  id        String   @id @default(cuid())
-  name      String   @unique
+  id        String      @id @default(cuid())
+  name      String      @unique
   nameEn    String?
-  slug      String   @unique
-  createdAt DateTime @default(now())
+  slug      String      @unique
+  createdAt DateTime    @default(now())
 
   // Relations
-  projects  Project[]
+  projects  ProjectTag[]
 
   // Indexes
   @@index([slug], map: "idx_tag_slug")
@@ -83,5 +83,21 @@ model ExternalLink {
   // Indexes
   @@index([projectId], map: "idx_link_projectId")
   @@index([type], map: "idx_link_type")
+  @@index([url], map: "idx_link_url")
+  @@index([type, url], map: "idx_link_type_url")
+  @@unique([projectId, url])
   @@map("external_links")
 }
+
+// Project-Tag many-to-many relationship table
+model ProjectTag {
+  projectId String
+  tagId     String
+
+  project   Project @relation(fields: [projectId], references: [id], onDelete: Cascade)
+  tag       Tag     @relation(fields: [tagId], references: [id], onDelete: Cascade)
+
+  @@id([projectId, tagId])
+  @@index([tagId])
+  @@map("project_tags")
+}