feat(provider): add monthly spending limits

[SHLE1]

Provider spending controls currently offer daily, weekly, and total limits. Add a monthly limit for API Key and Bedrock providers, with either a 30-day elapsed window or a fixed reset date, hour, and timezone. Fixed dates beyond a month's length reset on its final day and return to the configured date the following month.

Connect monthly limits to creation and editing, usage displays, scheduling snapshots and admission, transactional accounting and quota-crossing events, notifications, manual resets, and provider duplication. Configuration edits retain the latest usage counters. The settings use provider Extra fields. Document the reset and accounting rules in the routing and billing guide.

Validation:

• Go tests for the provider, billing, scheduler cache, DTO, and gateway provider packages.
• Formatting checks and the pre-push quick checks, including Go lint/tests and frontend lint, type checking, UI checks, and related tests.
• PostgreSQL integration test packages compile. Their runtime tests were not run locally because the Docker daemon is unavailable.