docs: update requirements.md §20.4 + architecture.md for SUB-003 auto-switch changes (#441)

[vybe]

Context

PR #508 (feat: subscription auto-switch on first failure) landed with two doc gaps vs the project's single source of truth.

Required Updates

1. docs/memory/requirements.md §20.4

The following are now stale:

• auto_switch_subscriptions default: OFF → ON (opt-out)
• Trigger threshold: 2+ consecutive errors → 1 error
• Trigger surface: was '429 handler only' → now includes 503 + auth-class errors

2. docs/memory/architecture.md services table

subscription_auto_switch.py entry should reflect:

• New public API: handle_subscription_failure() + is_auth_failure()
• Broadened trigger surface (chat proxy, task execution service, scheduler)
• Default-on behaviour

Acceptance Criteria

• requirements.md §20.4 updated with correct defaults and trigger conditions
• architecture.md services table updated for subscription_auto_switch.py

[AndriiPasternak31]

/claim

[github-actions]

Claimed by @AndriiPasternak31. Label status-in-progress added.

Next steps:

1. Create branch: feature/536-<short-slug>
2. Implement the feature
3. Open a PR with Fixes #536

[AndriiPasternak31]

/unclaim

[github-actions]

Issue unclaimed by @AndriiPasternak31. Available for pickup.