feat: auto-seed PR environments with SEED_ON_DEPLOY flag

Release command now runs migrations + seeds test users when
SEED_ON_DEPLOY=true. Tree seeding runs as a background task
on startup via HTTP API. Everything is idempotent and non-fatal.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/config.py

@@ -72,6 +72,9 @@ class Settings(BaseSettings):
         """Check if Stripe is configured."""
         return self.STRIPE_SECRET_KEY is not None and self.STRIPE_WEBHOOK_SECRET is not None
 
+    # Deployment – auto-seed test data on PR environments
+    SEED_ON_DEPLOY: bool = False
+
     # CORS - set FRONTEND_URL in production (e.g., https://patherly.up.railway.app)
     CORS_ORIGINS: list[str] = ["http://localhost:3000", "http://localhost:5173", "http://localhost:5174"]
     FRONTEND_URL: Optional[str] = None