fix: correct development setup and API documentation

[Portals]

The development setup in #971 selects Gradle 8 with Java 25, documents an API header the server ignores, omits Playwright browser installation, and incorrectly claims that ./gradlew build validates the database schema.

• Select gradle_9 from the existing Nix pin, matching the Gradle 9.5.1 wrapper and supporting Java 25.
• Document Authorization: pre-shared <api_key_id>:<api_key_token>.
• Install E2E dependencies and Chromium before running tests, clarify the required Gamma image, and use the version tag format published by CI.
• Validate entity mappings by starting the application against the development database, where Flyway migrations and Hibernate validation run.

Validation: git diff --check passed. Cross-checked the instructions against the authentication converter, application configuration, Makefile, E2E package scripts, CI workflow, and pinned nixpkgs Gradle definition. No application builds or E2E tests were run; Nix is not installed locally, so the shell was not evaluated.

Targets chore/basic-dx so these fixes can be incorporated into #971.