fix: prevent double-submit on upload and handle DB race on cdn insert

[kuu-26]

Two related fixes for the upload flow:

1. Client: FileUploadComponent double-submit guard
The Upload button and Enter key had no guard against double-firing. Added _isUploading flag that blocks repeated clicks and disables the button during upload.

2. Server: CdnBucketService race condition handling
The check-then-insert pattern (AnyAsync → AddAsync → SaveChangesAsync) has a TOCTOU race under concurrent uploads. Both requests pass the AnyAsync check, both try to insert, one gets a DbUpdateException on the PK. The old code caught generic Exception and logged it as an error even though it was a harmless duplicate.

Changes:

• Catch DbUpdateException specifically before the generic Exception handler
• Detach the failed entity from the DbContext so it doesn't corrupt future operations
• Only log as error if the row still doesn't exist after the race (genuinely unexpected)
• Generic Exception catch remains as a safety net for actual failures

This should eliminate the Sentry alerts for VALOUR-BACKEND-2B (Failed DbCommand INSERT into cdn_bucket_items) while keeping the upload flow working correctly.

[SigmaTel71]

For reference: fixes #1370.