test(students): add unit, integration and e2e tests for /students (#109)

- unit: fixture shapes, mock API (GET/POST/PUT/DELETE), mock DB operations
- integration: real DB CRUD via testDb (list, filter, create, get, update, delete)
- e2e: handler calls directly with mock FreshContext + real DB
  covers auth (employee vs non-employee), 400/403/404 cases
- adds test:e2e deno task and CI step
- adds tests/helpers/handler.ts with makeEmployeeContext, makeContextWithAffiliation,
  makeGetRequest, makeJsonRequest utilities

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

deno.json

@@ -13,6 +13,7 @@
     "test": "deno test -A --no-check tests/",
     "test:unit": "deno test -A --no-check tests/unit/",
     "test:integration": "deno test -A --no-check tests/integration/",
+    "test:e2e": "deno test -A --no-check tests/e2e/",
     "migrate": "node_modules/.bin/drizzle-kit migrate"
   },
   "lint": {