feat: alert condition DSL and multi-check quorum/debounce

[amondnet]

Context / Motivation

Gatus's condition engine is its key differentiator — conditions over status code, response time, response body, and certificate expiry, expressed declaratively in YAML. StatusBeam currently has a fixed notion of what "down" means.

The counterpart problem is false positives, which is the most heavily discussed pain point in upptime: upptime/upptime#553 (24 comments), upptime/upptime#364, upptime/upptime#287. A single failed probe from one location opening an incident and paging the on-call is what drives people off these tools.

Proposed scope

• A versioned YAML condition expression language evaluated in the Worker, covering status code, latency, response body / keyword match, and (with the SSL check) certificate expiry.
• Versioned so the schema can evolve without breaking existing configs.
• N-of-M consecutive-failure debounce before an incident is opened or a notification is sent, configurable per check.
• Quorum across multiple checks/regions as an input to the same decision.

Acceptance criteria

• A condition expression over status, latency, body/keyword, and cert expiry evaluates correctly and is covered by tests.
• An invalid or unsupported condition version fails config validation with a clear message.
• An N-of-M debounce prevents a single transient failure from opening an incident or notifying.
• Existing configs without conditions keep working unchanged.

Priority / Effort / Dependencies

• Priority: p3
• Effort: L (~1 week)
• Dependencies: none hard. Related to the TCP/SSL check work (certificate expiry as a condition input) and to multi-region probes (quorum input).