Fix live metadata cache progress reporting

[blurbery]

What changed

• report cumulative cache activity after each processing or discovery batch
• calculate live completion from terminal durable queue jobs divided by all queue jobs
• keep failed attempts distinct from terminal failures in the task message
• show fractional percentages on the task list, task detail, and Server Activity
• add backend and frontend regression coverage

Root cause

The cache task only published 100% when a run ended. While it was processing, the task manager retained the default 0%; the UI therefore had no real percentage to display. The run-level failure counter also represented attempts, which made retryable artwork failures look permanent.

Validation

• focused percentage and Server Activity tests pass
• TypeScript production build passes
• frontend formatting passes
• ESLint reports no errors (existing warnings remain)
• all changed Go files pass gofmt
• full backend tests and the production image build run in GitHub Actions