feat(web): shared Postgres read-through cache for MusicBrainz (ApiCache)

Reopening an artist (discography), an album (tracklist), or repeating a lookup
re-hit MusicBrainz every time with no cache and no throttle. Adds a generic
Postgres-backed read-through cache keyed on the LOGICAL operation (not the
request URL) so the worker and web app share entries.

- New ApiCache(key, json, fetchedAt) model + migration (generic name so the
  Last.fm browse cache can ride the same table).
- lib/apicache.ts cached(): read-through, TTL from fetchedAt (tunable, no
  migration), serve-stale-on-outage, never caches null.
- Wrap the high-level MB fns with logical keys + TTLs: discography &
  tracklists & artist-name 30d, searchReleaseGroup 7d, searchArtists 1d.
  Search keys normalized (trim+lowercase) to match the worker byte-for-byte.
- album-modal: a session-scoped Map<rgMbid,Track[]> so reopening is instant
  with zero network (complements the DB cache).
- Test setup clears ApiCache between tests. web 147 tests, tsc + build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/prisma/migrations/20260714090112_add_api_cache/migration.sql

@@ -0,0 +1,11 @@
+-- CreateTable
+CREATE TABLE "ApiCache" (
+    "key" TEXT NOT NULL,
+    "json" JSONB NOT NULL,
+    "fetchedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "ApiCache_pkey" PRIMARY KEY ("key")
+);
+
+-- CreateIndex
+CREATE INDEX "ApiCache_fetchedAt_idx" ON "ApiCache"("fetchedAt");

web/prisma/schema.prisma

@@ -182,3 +182,15 @@ model ScanWorkItem {
   @@unique([scanId, path])
   @@index([scanId, done, artist, album])
 }
+
+// Generic read-through cache for external API responses shared by web + worker (keyed on
+// the LOGICAL operation, not the request URL, so both processes share entries). TTL is
+// applied at read time from fetchedAt in code (no expiresAt column → tunable, no migration).
+// Named generically so the Last.fm browse cache can ride the same table.
+model ApiCache {
+  key       String   @id
+  json      Json
+  fetchedAt DateTime @default(now())
+
+  @@index([fetchedAt])
+}