feat: add Request and Job schema with migration

Adds the Prisma schema (Request/Job models, RequestStatus/JobState
enums), a Prisma client singleton at web/src/lib/db.ts, and the
init_request_job migration, applied and verified against Postgres.

Also publishes the db service's port (5432:5432) in docker-compose.yml
so host-run Prisma commands can reach localhost:5432 for local dev.

web/prisma/schema.prisma

@@ -0,0 +1,46 @@
+generator client {
+  provider = "prisma-client-js"
+}
+
+datasource db {
+  provider = "postgresql"
+  url      = env("DATABASE_URL")
+}
+
+enum RequestStatus {
+  pending
+  completed
+  needs_attention
+}
+
+enum JobState {
+  requested
+  matching
+  matched
+  downloading
+  tagging
+  imported
+  needs_attention
+}
+
+model Request {
+  id        String        @id @default(cuid())
+  artist    String
+  album     String
+  status    RequestStatus @default(pending)
+  createdAt DateTime      @default(now())
+  job       Job?
+}
+
+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
+}