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>

package.json

@@ -73,12 +73,14 @@
     "zod": "^3.22.4"
   },
   "devDependencies": {
+    "@types/autocannon": "^7.12.7",
     "@types/bcryptjs": "^2.4.6",
     "@types/jsonwebtoken": "^9.0.10",
     "@types/luxon": "^3.7.5",
     "@types/node": "^20.12.7",
     "@typescript-eslint/eslint-plugin": "^7.6.0",
     "@typescript-eslint/parser": "^7.6.0",
+    "autocannon": "^8.0.0",
     "eslint": "^8.57.0",
     "eslint-config-prettier": "^9.1.0",
     "husky": "^9.0.11",