DevContainer fixes

[colby-swandale]

This PR fixes up a few issues that was causing the DevContainer experience to not be up to scratch. Notable changes are some over Capybara configuration that needed cleaning up, and some tweaks to how we set the database, and dropping DATABASE_URL.

After this PR is merged, I can run spin up a DevContainer with everything pre-configured, and all tests are now passing. 🎉

[Copilot]

Pull request overview

This PR improves the DevContainer developer experience by simplifying system test networking/configuration and adjusting database configuration so the containerized services work without relying on DATABASE_URL.

Changes:

• Simplifies Capybara/WebMock configuration for DevContainer-driven test runs (removes served_by/env overrides and narrows WebMock allowlist).
• Updates config/database.yml.sample to support a containerized DB via DB_HOST instead of hardcoded localhost.
• Updates DevContainer environment variables and updates Playwright installation in bin/setup.

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/test_helper.rb	Removes DevContainer-specific host branching and tightens WebMock allowlist.
test/application_system_test_case.rb	Removes served_by override tied to CAPYBARA_SERVER_PORT.
config/database.yml.sample	Uses DB_HOST env var to point Rails at the containerized Postgres service.
bin/setup	Installs Playwright Chromium with --with-deps during setup.
.devcontainer/devcontainer.json	Drops DATABASE_URL/Capybara envs and sets DB_HOST for DB connectivity.
.devcontainer/devcontainer-lock.json	JSON formatting/structure normalization (no functional change).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.08%. Comparing base (0689682) to head (5f85a79).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6540      +/-   ##
==========================================
- Coverage   97.09%   97.08%   -0.01%     
==========================================
  Files         511      511              
  Lines       10775    10775              
==========================================
- Hits        10462    10461       -1     
- Misses        313      314       +1     

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