[➕ Feature]: Add official support for MariaDB as a database backend

Is your feature request related to a problem? Please describe. Keep documents and tests MySQL, PostgreSQL, and SQLite as supported backends. MariaDB isn't mentioned anywhere — no docs, no test coverage — even though it's commonly used as a MySQL-compatible replacement (e.g. in Galera Cluster HA setups).

We hit a concrete example of why this matters: our schema ended up mixing utf8mb4_0900_ai_ci (a MySQL 8–only collation) with utf8mb4_unicode_ci, causing Illegal mix of collations errors that broke alert ingestion and incident retrieval. Since MariaDB doesn't implement utf8mb4_0900_ai_ci, any migration relying on it would behave differently or fail on MariaDB.

Describe the solution you'd like Confirm MariaDB's support status, or explicitly state that only MySQL 8+ is supported. Add CI test coverage against MariaDB, similar to the existing MySQL/PostgreSQL matrix. Review model/migration defaults for collations or syntax that assume MySQL 8 specifically.

Describe alternatives you've considered Running Keep on MySQL 8 directly — works, but rules out environments standardized on MariaDB/Galera.

Additional context Happy to share more detail on the collation mismatch if useful for triag