feat: taste onboarding for new listeners (M6)

- GET /onboarding offers the library's top artists (with album covers)
  and genres and says whether to show it (not done and < 10 plays);
  PUT /me/onboarding stores the choice or a skip (migration 00009).
- The recommender seeds the taste from it while the user has fewer than
  30 scored tracks: chosen artists' tracks +1, chosen genres +0.5.
- Web: onboarding replaces the wave on the home page until done or
  skipped; artist grid with covers, genre chips, at least 3 artists.
- e2e journey and the recommender DB test cover it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/web/src/pages/onboarding.module.css

@@ -0,0 +1,124 @@
+.page {
+  display: flex;
+  flex-direction: column;
+  gap: 28px;
+  max-width: 1100px;
+  margin: 0 auto;
+  animation: pipuIn 0.2s ease-out;
+}
+
+.head h1 {
+  margin: 0 0 8px;
+  font-family: var(--font-display);
+  font-size: 32px;
+  font-weight: 600;
+  letter-spacing: -0.02em;
+}
+
+.head p {
+  margin: 0;
+  color: var(--text2);
+}
+
+.label {
+  margin: 0 0 14px;
+  font-size: 13px;
+  font-weight: 700;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  color: var(--text3);
+}
+
+.artists {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(120px, 1fr));
+  gap: 18px 12px;
+}
+
+.artist {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 10px;
+  padding: 10px 6px;
+  border-radius: var(--radius-lg);
+  text-align: center;
+}
+
+.artist:hover {
+  background: var(--hover);
+}
+
+.avatar {
+  position: relative;
+  border-radius: 50%;
+  transition:
+    box-shadow 0.15s,
+    transform 0.15s;
+}
+
+.on .avatar {
+  box-shadow:
+    0 0 0 3px var(--bg),
+    0 0 0 5px var(--accent);
+  transform: scale(0.96);
+}
+
+.check {
+  position: absolute;
+  right: -2px;
+  bottom: -2px;
+  display: grid;
+  place-items: center;
+  width: 32px;
+  height: 32px;
+  border-radius: 50%;
+  background: var(--accent);
+  color: #fff;
+  box-shadow: var(--glow);
+}
+
+.name {
+  font-size: 14px;
+  font-weight: 600;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  max-width: 100%;
+}
+
+.genres {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px;
+}
+
+.genre {
+  height: 38px;
+  padding: 0 16px;
+  border-radius: var(--radius-pill);
+  border: 1px solid var(--line);
+  background: var(--surface2);
+  font-weight: 600;
+  font-size: 14px;
+}
+
+.genre[aria-pressed='true'] {
+  background: var(--accent);
+  border-color: var(--accent);
+  color: #fff;
+  box-shadow: var(--glow);
+}
+
+.actions {
+  position: sticky;
+  bottom: 16px;
+  display: flex;
+  justify-content: flex-end;
+  gap: 10px;
+  padding: 12px;
+  border-radius: var(--radius-lg);
+  border: 1px solid var(--line2);
+  background: var(--glass);
+  backdrop-filter: blur(28px) saturate(1.3);
+}