Add podcast-only demo mode banner and router/health visibility

[AJaySi]

Motivation

• Make the podcast-only demo deployment mode immediately visible to operators via startup logs and API health/readiness endpoints.
• Allow operators and automation to verify which router groups were mounted or intentionally skipped when the demo flag is enabled.
• Ensure a clear boolean readiness signal podcast_only_demo_mode is exposed so deployment orchestration and alerts can react quickly.

Description

• Added an _env_flag_enabled helper and PODCAST_ONLY_DEMO_MODE constant in backend/app.py to detect ALWRITY_PODCAST_ONLY_DEMO_MODE / PODCAST_ONLY_DEMO_MODE.
• Exposed podcast_only_demo_mode in the /health and /health/readiness endpoints and extended /api/routers/status to include podcast_only_demo_mode and a router_groups map describing mounted/skipped groups and reasons.
• Added router_group_status bookkeeping and conditional router inclusion so core/optional/platform/advanced router groups are skipped when demo mode is enabled while keeping podcast and asset-serving routes mounted.
• Added a clear startup banner in backend/start_alwrity_backend.py that prints PODCAST-ONLY DEMO MODE ACTIVE (and context) when the demo flag is present.

Testing

• Compiled the modified files with python -m compileall backend/app.py backend/start_alwrity_backend.py, which completed successfully with no syntax errors.

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Codex Task

[AJaySi]

Merged