Add startup readiness checks and authenticated tenant readiness endpoint

[AJaySi]

Motivation

• Replace using init_database() as the sole readiness signal so startup validates filesystem and tenant DBs in multi-tenant mode.
• Make startup failures explicit and optionally fail-fast in production to avoid running in an unknowingly degraded state.
• Provide operators a per-tenant readiness check (under auth context) distinct from platform startup health.

Description

• Added a shared startup health module backend/services/startup_health.py that verifies the workspace root exists and is writable, validates DB open/create for single-tenant or a tenant (or synthetic) path in multi-tenant, and performs required schema/table+column compatibility checks for baseline migrations using _REQUIRED_SCHEMA (e.g., onboarding_sessions, daily_workflow_plans).
• Replaced init_database() as the primary readiness signal in both backend/main.py and backend/app.py by calling run_startup_health_routine() during application startup and logging errors explicitly, and re-raising to enable fail-fast behavior when configured.
• Added an authenticated readiness endpoint GET /health/readiness (in both entrypoints) that returns the latest startup report via get_startup_status() and a per-tenant auth-context check via readiness_under_auth_context() which resolves the tenant DB path and attempts a session/query.
• Added operator documentation docs/STARTUP_READINESS_BEHAVIOR.md describing single-tenant vs multi-tenant expectations, warning vs failure conditions, and the ALWRITY_FAIL_FAST_STARTUP / production default semantics.

Testing

• Compiled the modified modules with python -m py_compile backend/services/startup_health.py backend/main.py backend/app.py which succeeded.
• Executed the startup routine interactively via run_startup_health_routine() which exercised DB initialization paths and reported a failed status in this environment due to the workspace root not existing (this is environment-dependent and expected in a clean test container); the routine logged errors and warnings as designed.
• No unit tests were added; behaviour validated via static compile and a runtime invocation of run_startup_health_routine().

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Codex Task