Add public demo mode with auto-seeding, hourly reset, and Portainer deploy guide

- DEMO_MODE=true env flag: disables password changes and backup endpoints (403),
  exposes GET /demo/status for frontend detection
- Auto-seed on first startup: creates demo user (demo@mymidas.app / demo123)
  with 6 months of transactions, investments, budgets, subscriptions, and tax
  payslips; takes a pg_dump snapshot immediately after for hourly restore
- Hourly reset: resetter Alpine container with cron restores DB from snapshot
  and purges uploaded attachments every hour on the hour
- Frontend: amber demo banner on all pages, login page shows credentials,
  password change disabled with notice, backups section replaced with notice
- demo/ directory: self-contained docker-compose.yml (ports 4001/8091),
  .env.example, reset.sh, and step-by-step Portainer DEPLOY.md

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

backend/app/config.py

@@ -14,6 +14,7 @@ class Settings(BaseSettings):
     environment: str = "production"
     allow_registration: bool = False
     base_currency: str = "GBP"
+    demo_mode: bool = False
 
     # JWT — keys read from /run/secrets/ at runtime
     jwt_private_key_file: str = "/run/secrets/jwt_private.pem"
@@ -48,6 +49,10 @@ class Settings(BaseSettings):
     def is_development(self) -> bool:
         return self.environment == "development"
 
+    @property
+    def is_demo(self) -> bool:
+        return self.demo_mode
+
 
 @lru_cache
 def get_settings() -> Settings: