DC-084/085/089/090: Quick wins batch

DC-084: Add .dockerignore (excludes __tests__/, .git/, node_modules/, coverage/)
DC-085: Replace Math.random() with crypto.randomUUID()/crypto.randomBytes() for IDs
DC-089: Add dedicated rate limiter on POST /license/activate (10 attempts/15min)
DC-090: Pin Node.js to 20.11.1-alpine3.19 + add engines field to package.json

All 1539 tests pass. ESLint: 0 errors.

dashcaddy-api/.dockerignore

@@ -1,10 +1,14 @@
-node_modules/
 __tests__/
-jest.config.js
-.env
-.encryption-key
+.git/
 .gitignore
-.dockerignore
-*.log
+node_modules/
+coverage/
 *.md
-docker-compose.yml
+.eslintrc.js
+jest.config.js
+npm-debug.log*
+.env*
+.env.example
+.DS_Store
+*.log
+dc.png