Guard onboarding registration in podcast-only demo mode

[AJaySi]

Motivation

• Provide a safe demo mode that disables onboarding behavior for podcast-only demos while keeping onboarding code present for full environments.
• Prevent frontend probe failures by returning a lightweight controlled response when onboarding is intentionally disabled.

Description

• Add PODCAST_ONLY_DEMO_MODE (env-driven truthy parsing) in backend/app.py to control demo behavior at startup.
• Replace unconditional OnboardingManager(app) with a guarded initialization that sets onboarding_manager = None and only creates OnboardingManager(app) when PODCAST_ONLY_DEMO_MODE is false.
• Keep the /api/onboarding/status endpoint available and return a fixed demo payload when PODCAST_ONLY_DEMO_MODE is enabled to avoid client errors ({"enabled": false, "status": "disabled", "message": "Onboarding is disabled for podcast-only demo mode.", "demo_mode": "podcast_only"}).
• Do not remove or delete onboarding modules; only bypass runtime registration so full functionality returns when the flag is off.

Testing

• Ran python -m py_compile backend/app.py which completed successfully (no syntax errors).

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Codex Task