project improvements — config, tests, refactoring, and new features

[zinchuknadia]

Changes included in this PR:

• Remove VK and Yandex social login providers
  VK and Yandex OAuth providers were removed as they are no longer needed.
  This cleans up unused dependencies and configuration.

• Move sensitive config (DB, OAuth, mail) to env variables
  Extracted sensitive properties (DB credentials, OAuth IDs/secrets, mail settings)
  into a separate config file. Values are now read from environment variables at startup.

• Switch tests to H2 in-memory DB with Spring profile selection
  Replaced PostgreSQL with H2 in-memory database for test execution.
  Two datasource beans are defined; the active Spring profile determines which one is used.

• Add tests for ProfileRestController (success and failure paths)
  Added coverage for all public methods of ProfileRestController.
  Both success and unsuccessful paths are tested.

• Refactor FileUtil#upload to use modern NIO API
  Replaced legacy File-based file handling with java.nio.file (Path and Files).
  This aligns the code with modern Java best practices.

• Add task time tracking (in-progress and testing duration)
  Added two service methods to calculate time spent in development and testing stages.
  Also added seed ACTIVITY records to changelog.sql with in_progress, ready_for_review, and done statuses.

• Add Dockerfile for the application server
  Added a Dockerfile to containerize the Spring Boot application.
  Enables consistent builds and easier deployment.

• Add docker-compose for server, database, and nginx
  Added docker-compose.yml to run the application server together with PostgreSQL and nginx.
  Used config/nginx.conf as the nginx configuration file.