fix(runway): ISS-010 stop logging queue credentials

[sbalabanov]

Summary

Intent

• Prevent QUEUE_MYSQL_DSN credentials and query parameters from appearing in Runway startup logs.
• Preserve a useful queue-initialization signal without rendering connection configuration.

Changes

• Log the queue backend instead of the raw DSN.
• Add observer-based regression coverage with sentinel username, password, and query values.

Reproduction

Before the fix, starting Runway with QUEUE_MYSQL_DSN=user:super-secret@tcp(mysql:3306)/submitqueue?parseTime=true produced initialized queue {"dsn":"user:super-secret@tcp(mysql:3306)/submitqueue?parseTime=true"}.

After the fix, the same configuration produces initialized queue {"backend":"mysql"} with no DSN components.

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