fix: prevent concurrent plugin install conflicts - #794

Description

• atomically create and lock shared plugin records before updating installation state
• keep tenant installations idempotent and plugin reference counts consistent under concurrency
• cover concurrent installs for the same tenant and across independent tenants

Type of Change

• Bug fix
• New feature
• Refactor
• Performance improvement
• Other

Essential Checklist

Testing

• I have tested the changes locally and confirmed they work as expected
• I have added unit tests where necessary and they pass successfully

Bug Fix (if applicable)

• I have used GitHub syntax to close the related issue (e.g., Fixes #123 or Closes #123)

Additional Information

Tracked in Linear: SNP-623