feat(read-models): add projection_offset table + idempotency harness (GOO-187)

RFC-003 §0 Phase 0 — the (eventId, handlerName) offset contract.

- prisma: ProjectionOffset model + 20260424100000_add_projection_offset
  migration (composite PK, handlerName/appliedAt index for reconciliation)
- infra: PrismaProjectionOffsetStore (createMany skipDuplicates +
  applyWithOffset transactional helper that rolls offset+mutation back
  together on failure)
- module: bind PROJECTION_OFFSET_STORE → PrismaProjectionOffsetStore
- testing: assertProjectorIdempotent harness ("replay N times → single
  state mutation") for Phase 2/3 projector specs to reuse
- tests: 12 specs lock the contract — replay 5x → 1 mutation, broken
  projectors fail loudly, two projectors keep independent offsets

Note: prisma format normalised existing column alignment when the new
model was added; the meaningful diff is the appended ProjectionOffset
block at the bottom of schema.prisma.

Acceptance criteria from issue:
- migration applies cleanly (validated via prisma format/parse)
- harness exported from read-models/testing and used by example
  projector spec at __tests__/idempotency-harness.spec.ts

Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/migrations/20260424100000_add_projection_offset/migration.sql

@@ -0,0 +1,23 @@
+-- RFC-003 Phase 0 (GOO-187): projection_offset table.
+--
+-- Idempotency contract for CQRS projectors. Every projector dispatch
+-- wraps `apply()` in a transaction that inserts (event_id, handler_name)
+-- here. Re-deliveries hit the composite primary key, roll back, and the
+-- projector observes a no-op.
+--
+-- Port:             apps/api/src/modules/read-models/domain/projection-offset-store.ts
+-- Prisma adapter:   apps/api/src/modules/read-models/infrastructure/prisma-projection-offset-store.ts
+-- Test harness:     apps/api/src/modules/read-models/testing/
+
+-- CreateTable
+CREATE TABLE "projection_offset" (
+    "eventId" TEXT NOT NULL,
+    "handlerName" TEXT NOT NULL,
+    "appliedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "payloadHash" TEXT,
+
+    CONSTRAINT "projection_offset_pkey" PRIMARY KEY ("eventId", "handlerName")
+);
+
+-- CreateIndex (handler-scoped scans for reconciliation tooling)
+CREATE INDEX "projection_offset_handlerName_appliedAt_idx" ON "projection_offset"("handlerName", "appliedAt" DESC);