fix(prometheus): add Thanos Querier compatibility mode

[ykd007]

Problem

Connecting Keep's Prometheus provider to a Thanos Querier fails with:

prometheus scopes are invalid: { "connectivity": "'NoneType' object is not iterable" }

This happens because Thanos Querier does not implement /api/v1/alerts (that endpoint only exists on Prometheus's built-in rules evaluator / Alertmanager). Keep's connectivity check and alert polling both hit that endpoint unconditionally.

Closes #6474

Solution

Adds a thanos_compatible: bool flag (default False) to PrometheusProviderAuthConfig. When enabled:

• Connectivity check uses /api/v1/rules?type=alert instead of /api/v1/alerts
• Alert polling uses /api/v1/rules?type=alert which Thanos supports and which returns full alert data including annotations, state (firing/pending), activeAt, and labels
• Fingerprints are computed as MD5 of sorted label JSON (same approach used elsewhere in the codebase)
• Fully backward compatible — existing Prometheus configs with thanos_compatible: false (default) are unchanged

Testing

Set thanos_compatible: true in provider auth config when connecting to a Thanos Querier URL.

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