feat(api): add blend accrue keeper

[Adecom16]

Closes #466

Summary

• Add a scheduled Blend accrual keeper.
• Discover adapters dynamically and submit accrue() only for Blend-backed adapters.
• Add retry handling and structured failure reporting.
• Add a protected cron endpoint and configuration.
• Add tests and operational documentation.

Validation

• Tests passed
• Lint passed
• Build passed

[vercel]

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[collinsezedike]

CI is failing on Test, not for a failing test (all 117 pass) but on the coverage threshold: Coverage for branches (73.95%) does not meet global threshold (76%).

accrual-keeper.ts is at 64.04% branch coverage, with 120, 126, 131, 153, 191-198, 330-375, 396-399, 405-413 uncovered. That's the retry and failure-handling logic specifically, the part covering what happens when a submission fails, which is exactly what determines whether this keeper degrades safely or silently. Please add tests covering those branches so the coverage threshold passes.