Uncertainty engine: calibration roll cron + Brier/reliability SQL

[ssilvius]

Phase 1 of the uncertainty engine spike. See docs/designs/uncertainty-engine.md.

Scope:

• Nightly cron handler
• For each active cohort_key, recompute uncertainty_calibration_snapshot rows
• 10 buckets of 0.1 width on claimed_confidence
• Per bucket: mean claimed, mean correctness, count, orphan count, Brier score
• Replace prior snapshot rows for the cohort atomically
• Wire into wrangler.jsonc triggers

Dependency: orphan sweep issue (state machine working).

Tests: .test.ts on the Brier math and bucketing logic with deterministic fixtures (no D1).