Add Concert Finder and Rabbit Hole features

- Concert Finder: Bandsintown API integration to find upcoming shows for
  recommended artists, with expandable tour dates section on recommendation cards
- Rabbit Hole: Multi-step guided discovery journey where each song connects
  to the next through a shared musical quality (producer, influence, tone, etc.)
- New /rabbit-hole route with seed input, step count selector, and vertical
  chain visualization of connected songs
- Added concerts endpoint, rabbit hole endpoint, and corresponding frontend
  API functions and navigation

frontend/src/components/Layout.tsx

@@ -1,6 +1,6 @@
 import { useState } from 'react'
 import { Link, useLocation, useNavigate } from 'react-router-dom'
-import { Disc3, LayoutDashboard, Fingerprint, Clock, ListMusic, ListPlus, Compass, Lightbulb, Store, Users, Heart, Crown, Shield, Menu, X, LogOut, User } from 'lucide-react'
+import { Disc3, LayoutDashboard, Fingerprint, Clock, ListMusic, ListPlus, Compass, Lightbulb, Store, Users, ArrowDownCircle, Heart, Crown, Shield, Menu, X, LogOut, User } from 'lucide-react'
 import { useAuth } from '../lib/auth'
 
 const ADMIN_EMAIL = 'chris.ryan@deepcutsai.com'
@@ -14,6 +14,7 @@ const baseNavItems = [
   { path: '/analyze', label: 'Analyze', icon: Lightbulb },
   { path: '/generate-playlist', label: 'Create Playlist', icon: ListPlus },
   { path: '/crate', label: 'Crate Dig', icon: Disc3 },
+  { path: '/rabbit-hole', label: 'Rabbit Hole', icon: ArrowDownCircle },
   { path: '/bandcamp', label: 'Bandcamp', icon: Store },
   { path: '/compatibility', label: 'Taste Match', icon: Users },
   { path: '/saved', label: 'Saved', icon: Heart },