feature: improve deletion speed and multi cluster handling by using background sync

[thoro]

Disclaimer: This code was generated by Opus 4.5 based on my ideas and prompt. It was tested with some test databases in a k8s cluster, but I do lack the full understanding of the code to say this is a good change / idea.

This is based on my second PR #3015 and further improves how quickly dbs are deleted after they are marked. I specifically ran into these issues when trying to restore from a backup, and the secrets inside of k8s run out of sync. This makes the operator extremly slow and unresponsive, therefore the idea of syncing each database in it's own go routine - most of the code anyways usually waits on network i/o.

Here is what was adapted:

    Improve sync responsiveness with background execution and context cancellation

    This change improves the responsiveness of the operator when handling
    deletion requests by running sync operations in the background and
    using context cancellation to interrupt stuck operations.

    Changes:
    - Add context field to Cluster struct, passed through New()
    - Add Cancel() method to cancel cluster's context
    - Add StartSync/EndSync/NeedsResync for managing background sync state
    - Run Sync() in a background goroutine so worker can process other events
    - Add context-aware DB connection methods (initDbConnWithContext)
    - Add RetryWithContext() that respects context cancellation
    - Cancel cluster context immediately when DeletionTimestamp detected
    - Use context-aware connections in syncRoles/syncDatabases

    Flow:
    1. Sync event spawns background goroutine and returns immediately
    2. If another sync arrives while one is running, needsResync flag is set
    3. When sync completes, it checks needsResync and requeues if needed
    4. Delete cancels context -> stuck DB operations return early -> mutex released
    5. Delete proceeds without waiting for slow/stuck sync operations    

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