Grader x verdictBucket grade-tendency views to drive MARS-504 backfill coverage trigger

Backfill MARS-504 needs a grader x verdictBucket grade-tendency cross-tab to drive its coverage trigger; no existing 070 view provides it (vPracticaVerdictPath has verdictBucket+finalCalificacion but not the grader; vGraderLeaderboard has grader but only counts). Build two read-only reporting views: (1) vGraderBucketGradeTendency = one row per (graderUserId, verdictBucket): gradedCount, medianCalificacion, min/max calif, graderActive — grader = reviewedBy of the LATEST review on the práctica, over finalGraded=true rows only. (2) vBucketGradeTendency = per verdictBucket cohort rung: gradedCount, distinctGraders, medianCalificacion — the fallback ladder's cohort tier. Grants: service_role + evolutiva_agent_ro/rw only, terminal DO-block asserting no anon/authenticated/PUBLIC (match migration 070). Class-A migration by db-mars-cc, audit design-ping + PASS, then db:export bundle (schema.md+reference-data.tsv). Purpose: read the coverage trigger (count grader-bucket cells with gradedCount>=threshold AND distinctGraders>=2) to decide when the per-teacher backfill model is representative.