feat: Initial backend setup for Music Analyser

- Created FastAPI backend structure.
- Implemented Spotify Recently Played ingestion logic.
- Set up SQLite database with SQLAlchemy models.
- Added AI Service using Google Gemini.
- Created helper scripts for auth and background worker.
- Added Dockerfile and GitHub Actions workflow.

backend/app/services/spotify_client.py

@@ -0,0 +1,70 @@
+import os
+import base64
+import time
+import httpx
+from fastapi import HTTPException
+
+SPOTIFY_TOKEN_URL = "https://accounts.spotify.com/api/token"
+SPOTIFY_API_BASE = "https://api.spotify.com/v1"
+
+class SpotifyClient:
+    def __init__(self, client_id: str, client_secret: str, refresh_token: str):
+        self.client_id = client_id
+        self.client_secret = client_secret
+        self.refresh_token = refresh_token
+        self.access_token = None
+        self.token_expires_at = 0
+
+    async def get_access_token(self):
+        """Returns a valid access token, refreshing if necessary."""
+        if self.access_token and time.time() < self.token_expires_at:
+            return self.access_token
+
+        print("Refreshing Spotify Access Token...")
+        async with httpx.AsyncClient() as client:
+            auth_str = f"{self.client_id}:{self.client_secret}"
+            b64_auth = base64.b64encode(auth_str.encode()).decode()
+
+            response = await client.post(
+                SPOTIFY_TOKEN_URL,
+                data={
+                    "grant_type": "refresh_token",
+                    "refresh_token": self.refresh_token,
+                },
+                headers={"Authorization": f"Basic {b64_auth}"},
+            )
+
+            if response.status_code != 200:
+                print(f"Failed to refresh token: {response.text}")
+                raise Exception("Could not refresh Spotify token")
+
+            data = response.json()
+            self.access_token = data["access_token"]
+            # expires_in is usually 3600 seconds. buffer by 60s
+            self.token_expires_at = time.time() + data["expires_in"] - 60
+            return self.access_token
+
+    async def get_recently_played(self, limit=50):
+        token = await self.get_access_token()
+        async with httpx.AsyncClient() as client:
+            response = await client.get(
+                f"{SPOTIFY_API_BASE}/me/player/recently-played",
+                params={"limit": limit},
+                headers={"Authorization": f"Bearer {token}"},
+            )
+            if response.status_code != 200:
+                print(f"Error fetching recently played: {response.text}")
+                return []
+
+            return response.json().get("items", [])
+
+    async def get_track(self, track_id: str):
+        token = await self.get_access_token()
+        async with httpx.AsyncClient() as client:
+            response = await client.get(
+                f"{SPOTIFY_API_BASE}/tracks/{track_id}",
+                headers={"Authorization": f"Bearer {token}"},
+            )
+            if response.status_code != 200:
+                return None
+            return response.json()