feat(tests): add Jest+Supertest infrastructure with jump route suite (14 tests)

- Extract Express app creation into src/createApp.js to enable testing without starting the server
- Add Jest, Supertest, test helpers (createTestApp, auth token generator)
- Write 14 integration tests for skydive/jumps: auth protection, CRUD, ownership checks

Bugs caught and fixed by the test suite:
- MongoDB models: author field typed as Schema.Types.UUID (Mongoose 6 rejects string UUIDs) → changed to String
- Jump.toJSONFor: called toProfileJSONFor on a UUID string → guarded with typeof check
- exceptions.handler: used err.statusCode but express-jwt throws err.status → returns 500 on 401 errors
- Jump.count(query): deprecated Mongoose method was ignoring the filter → replaced with countDocuments
- jumps.routes GET /: $or query included non-schema field "title" stripped by strictQuery, leaving {} (match-all) → replaced with slug/lieu search
- $regex: was passing a JS RegExp object which serializes to {} in MongoDB → now passes raw string

tests/jest.env.js

@@ -0,0 +1,2 @@
+process.env.APP_ENV = 'test';
+require('dotenv').config({ path: 'config/env/.env.test' });