Promote feedback mechanism from jsonb blob to first-class column on appLog tables (fleet)

Elazar 2026-07-07: filtering the feedback MECHANISM sub-discriminator (toast|guardrail|action-error|native-validation|client-validation) currently requires a jsonb blob traversal (detail->>'mechanism') — suboptimal for a primary query axis. Promote mechanism to a first-class indexed COLUMN on appEvents (and mirror across all appLog tables fleet-wide: mars/pluto/venus). NOTE: category + action are ALREADY real columns — the main cuts (category='toast', action='error-shown' vs 'toast-shown') are plain column filters, NOT blob. Only the mechanism refinement is in jsonb. Scope: add nullable 'feedbackMechanism' column + backfill from detail.mechanism + repoint emitters/digest to the column; DDL → team/db-venus. Class-A (schema). Fleet-coordinate via aro:evolutiva-management.