Implement acme.sh-based certificate revocation lifecycle

[streaky]

This pull request adds durable, idempotent support for revoking issued certificates at their ACME certificate authority, including API, backend, database, and documentation changes. It introduces a new /v1/certificates/{id}/revoke endpoint, persists revocation requests and their statuses, and ensures that revocation is handled safely and consistently, including error handling and idempotency via an Idempotency-Key.

API and Documentation:

• Added a new POST /v1/certificates/{id}/revoke endpoint for revoking a certificate at its CA, with support for RFC 5280 reason codes and idempotency. (README.md, acme_api/routers/certificates.py, [1] [2] [3]
• Updated documentation to clarify the difference between local deletion (DELETE) and CA revocation (POST /revoke). (README.md, README.mdR199-R220)

Database and Models:

• Introduced a new certificate_revocations table and corresponding SQLAlchemy model to persist revocation requests, their status, and metadata. (acme_api/models/certificate_revocation.py, alembic/versions/a7c3e9d2b4f1_add_certificate_revocations.py, [1] [2]
• Linked revocations to certificates via a new relationship. (acme_api/models/certificate.py, acme_api/models/certificate.pyR178-R183)

Backend and Business Logic:

• Implemented backend support for invoking acme.sh to perform the revocation, including error handling and idempotency guarantees. (acme_api/backend/acmesh_backend.py, acme_api/services/certificate_revocations.py, [1] [2]
• Extended the backend protocol and mock backend to support the new revocation operation. (acme_api/backend/protocol.py, acme_api/backend/mock_backend.py, [1] [2]

Schemas and Validation:

• Added Pydantic schemas for revocation requests and responses, including validation to reject unused RFC 5280 reason codes. (acme_api/schemas/certificate.py, acme_api/schemas/certificate.pyR73-R103)

Testing Support:

• Enhanced test helpers to track and control revocation operations for backend testing. (tests/helpers/api.py, [1] [2] [3]