Uncertainty engine: Drizzle schema + zod companions

[ssilvius]

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

Scope:

• apps/web/src/db/schema/uncertainty.ts -- Drizzle table definitions for uncertainty_prediction and uncertainty_calibration_snapshot
• apps/web/src/db/schema/uncertainty.zod.ts -- insert/select schemas via drizzle-zod
• Match the migration column shapes exactly

Dependency: D1 migration issue must land first.

Convention reminder: every schema file gets a .zod.ts companion. No barrel files.