build(016-load-concurrency-testing): T001-T002 setup + concurrency-guard migration

Adds autocannon as a devDependency for the load-test tooling (T001), and the
shared schema migration T002 blocks: SLARun.version for optimistic
concurrency, plus two Postgres partial unique indexes
(assignments_one_current_per_ticket, escalation_events_ticket_rule_unique)
guarding against the assignment and escalation races research.md documents.
Applied directly to both the real dev DB and the throwaway test DB.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

prisma/migrations/20260909120000_add_concurrency_guards/migration.sql

@@ -0,0 +1,13 @@
+-- AlterTable
+ALTER TABLE "sla_runs" ADD COLUMN     "version" INTEGER NOT NULL DEFAULT 0;
+
+-- 016-load-concurrency-testing research.md §1: at most one current assignment per ticket,
+-- enforced at the database level (a partial unique index, since Prisma's schema DSL cannot
+-- express a WHERE-predicated unique constraint directly).
+CREATE UNIQUE INDEX "assignments_one_current_per_ticket" ON "assignments"("ticketId") WHERE "isCurrent" = true;
+
+-- 016-load-concurrency-testing research.md §3: a given escalation rule may fire at most once
+-- per ticket over that ticket's lifetime (SLARun.ticketId is already @unique — no reopen-cycle
+-- support, so a rule-triggered breach genuinely cannot recur for the same ticket). Manual
+-- escalations (rule_id IS NULL) are excluded and remain repeatable.
+CREATE UNIQUE INDEX "escalation_events_ticket_rule_unique" ON "escalation_events"("ticketId", "ruleId") WHERE "ruleId" IS NOT NULL;

prisma/schema.prisma

@@ -595,6 +595,11 @@ model SLARun {
 
   completedAt DateTime?
 
+  // 016-load-concurrency-testing: optimistic-concurrency counter, identical convention to
+  // Ticket.version (003-ticketing) — guards pause/resume/complete/the breach sweep against
+  // racing each other and silently clobbering this run's state (research.md §2).
+  version Int @default(0)
+
   @@index([status, resolutionDueAt])
   @@index([status, firstResponseDueAt])
   @@map("sla_runs")