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/migrations/20260710164008_add_candidate_library_item/migration.sql

@@ -0,0 +1,42 @@
+-- CreateTable
+CREATE TABLE "Candidate" (
+    "id" TEXT NOT NULL,
+    "jobId" TEXT NOT NULL,
+    "source" TEXT NOT NULL,
+    "format" TEXT NOT NULL,
+    "qualityClass" INTEGER NOT NULL,
+    "trackCount" INTEGER NOT NULL,
+    "confidence" DOUBLE PRECISION NOT NULL,
+    "sourceRef" TEXT NOT NULL,
+    "chosen" BOOLEAN NOT NULL DEFAULT false,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "Candidate_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "LibraryItem" (
+    "id" TEXT NOT NULL,
+    "requestId" TEXT NOT NULL,
+    "artist" TEXT NOT NULL,
+    "album" TEXT NOT NULL,
+    "path" TEXT NOT NULL,
+    "source" TEXT NOT NULL,
+    "format" TEXT NOT NULL,
+    "qualityClass" INTEGER NOT NULL,
+    "importedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "LibraryItem_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "LibraryItem_requestId_key" ON "LibraryItem"("requestId");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "LibraryItem_artist_album_key" ON "LibraryItem"("artist", "album");
+
+-- AddForeignKey
+ALTER TABLE "Candidate" ADD CONSTRAINT "Candidate_jobId_fkey" FOREIGN KEY ("jobId") REFERENCES "Job"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "LibraryItem" ADD CONSTRAINT "LibraryItem_requestId_fkey" FOREIGN KEY ("requestId") REFERENCES "Request"("id") ON DELETE CASCADE ON UPDATE CASCADE;

web/prisma/schema.prisma

@@ -30,6 +30,7 @@ model Request {
   status      RequestStatus @default(pending)
   createdAt   DateTime      @default(now())
   job         Job?
+  libraryItem LibraryItem?
 }
 
 model Job {
@@ -43,4 +44,34 @@ model Job {
   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
+  artist       String
+  album        String
+  path         String
+  source       String
+  format       String
+  qualityClass Int
+  importedAt   DateTime @default(now())
+
+  @@unique([artist, album])
 }