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

backend/app/api/endpoints/concerts.py

@@ -0,0 +1,42 @@
+from urllib.parse import quote
+
+import httpx
+from fastapi import APIRouter, Depends, Query
+
+from app.core.security import get_current_user
+from app.models.user import User
+
+router = APIRouter(prefix="/concerts", tags=["concerts"])
+
+
+@router.get("")
+async def find_concerts(
+    artist: str = Query(...),
+    user: User = Depends(get_current_user),
+):
+    """Find upcoming concerts for an artist using Bandsintown API."""
+    url = f"https://rest.bandsintown.com/artists/{quote(artist)}/events"
+    async with httpx.AsyncClient(timeout=10) as client:
+        resp = await client.get(url, params={"app_id": "vynl", "date": "upcoming"})
+
+    if resp.status_code != 200:
+        return {"events": []}
+
+    events = resp.json()
+    if not isinstance(events, list):
+        return {"events": []}
+
+    return {
+        "artist": artist,
+        "events": [
+            {
+                "date": e.get("datetime", ""),
+                "venue": e.get("venue", {}).get("name", ""),
+                "city": e.get("venue", {}).get("city", ""),
+                "region": e.get("venue", {}).get("region", ""),
+                "country": e.get("venue", {}).get("country", ""),
+                "url": e.get("url", ""),
+            }
+            for e in events[:10]
+        ],
+    }