feat: add Candidate and LibraryItem schema

Adds Candidate (per-job download candidates found by the pipeline)
and LibraryItem (per-request imported album, deduped on
artist+album) tables. LibraryItem.requestId is @unique since
Request.libraryItem is a one-to-one back-relation (required for
Prisma validation, matching the existing Job.requestId pattern).

web/prisma/schema.prisma

@@ -24,23 +24,54 @@ enum JobState {
 }
 
 model Request {
-  id        String        @id @default(cuid())
-  artist    String
-  album     String
-  status    RequestStatus @default(pending)
-  createdAt DateTime      @default(now())
-  job       Job?
+  id          String        @id @default(cuid())
+  artist      String
+  album       String
+  status      RequestStatus @default(pending)
+  createdAt   DateTime      @default(now())
+  job         Job?
+  libraryItem LibraryItem?
 }
 
 model Job {
+  id           String      @id @default(cuid())
+  request      Request     @relation(fields: [requestId], references: [id], onDelete: Cascade)
+  requestId    String      @unique
+  state        JobState    @default(requested)
+  currentStage String      @default("intake")
+  attempts     Int         @default(0)
+  error        String?
+  claimedAt    DateTime?
+  createdAt    DateTime    @default(now())
+  updatedAt    DateTime    @updatedAt
+  candidates   Candidate[]
+}
+
+model Candidate {
+  id           String   @id @default(cuid())
+  job          Job      @relation(fields: [jobId], references: [id], onDelete: Cascade)
+  jobId        String
+  source       String
+  format       String
+  qualityClass Int
+  trackCount   Int
+  confidence   Float
+  sourceRef    String
+  chosen       Boolean  @default(false)
+  createdAt    DateTime @default(now())
+}
+
+model LibraryItem {
   id           String   @id @default(cuid())
   request      Request  @relation(fields: [requestId], references: [id], onDelete: Cascade)
   requestId    String   @unique
-  state        JobState @default(requested)
-  currentStage String   @default("intake")
-  attempts     Int      @default(0)
-  error        String?
-  claimedAt    DateTime?
-  createdAt    DateTime @default(now())
-  updatedAt    DateTime @updatedAt
+  artist       String
+  album        String
+  path         String
+  source       String
+  format       String
+  qualityClass Int
+  importedAt   DateTime @default(now())
+
+  @@unique([artist, album])
 }