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>

.gitea/workflows/test.yml

@@ -68,3 +68,12 @@ jobs:
           POSTGRES_PASS: test
           POSTGRES_DB: polympr_test
         run: deno task test:integration
+
+      - name: Run e2e tests
+        env:
+          POSTGRES_HOST: 127.0.0.1
+          POSTGRES_PORT: 5432
+          POSTGRES_USER: test
+          POSTGRES_PASS: test
+          POSTGRES_DB: polympr_test
+        run: deno task test:e2e