Fix: rename db -> fitness-db to avoid DNS collision with Coolify's postgres

ROOT CAUSE FOUND: 'db' resolved to multiple IPs on the Docker network.
Coolify's own PostgreSQL (or another stack's) also responds to 'db'
via the predefined network. Our backend was connecting to the WRONG
postgres — one that requires password auth.

Fix: rename service from 'db' to 'fitness-db' so the hostname is unique.

docker-compose.yml

@@ -16,7 +16,7 @@ services:
     build: ./backend
     restart: unless-stopped
     environment:
-      - DATABASE_URL=postgresql+asyncpg://fitness@db:5432/fitness_rewards
+      - DATABASE_URL=postgresql+asyncpg://fitness@fitness-db:5432/fitness_rewards
       - SECRET_KEY=8915f64daf616b1b1ea5cdc8242ec924adf263c045bef378d9cb880787ba4386
       - STRAVA_CLIENT_ID=211907
       - STRAVA_CLIENT_SECRET=d1e48251327a3760acd6dcf4129ec59350dc6b71
@@ -24,7 +24,7 @@ services:
       - POLAR_CLIENT_SECRET=29890119-2267-4a72-8888-e67ab0c0bf60
       - BASE_URL=https://fitness.littlefake.com
     depends_on:
-      db:
+      fitness-db:
         condition: service_healthy
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:8000/api/health"]
@@ -33,7 +33,7 @@ services:
       retries: 10
       start_period: 30s
 
-  db:
+  fitness-db:
     image: postgres:16-alpine
     restart: unless-stopped
     environment: