Make enrichment backlog durable across quota windows

[batbrainy]

Problem

Enrichment demand can exceed the 40-request hourly allowance. Quota exhaustion must defer
entity work to later windows, not convert delay into a terminal outcome. The previous
skipped_budget and age-out path discarded enrichment intent even though actor and
repository rows already provided a durable work record.

Required behavior

• Keep pending and retryable_failure entities actionable until enrichment succeeds or
  establishes an entity-specific terminal outcome.
• Select the oldest backlog row first within each entity class across quota windows.
• Give durable backlog work priority within the enrichment allowance, with 20/20
  actor/repository guarantees and borrowing.
• Attempt refreshes only after the never-enriched backlog is observed empty.
• Remove skipped_budget and migrate historical skipped rows back into actionable states.
• Publish backlog size, oldest wait, claimability, and allowance usage through /status.
• Keep polling/control work isolated from quota-paced enrichment queue depth.

Acceptance criteria

• Quota denial leaves the entity durable and retryable in a later window.
• Backlog order survives restarts and window rollover.
• Historical skipped rows are restored without losing prior failure context.
• Tests cover rollover, FIFO ordering, borrowing, retry/backoff, recovery, queue isolation,
  and status reporting.
• Documentation describes backlog growth as the capacity limitation rather than dropped
  coverage.

Implementation

Tracked by PR #43.