[Server][Tests] Add first unit tests for config normalization via context

Summary Introduce the first server-side unit tests to validate configuration loading and normalization using the new createWikiContext(), without relying on global state.

Background

• Config and env flag normalization is currently scattered and relies on implicit truthiness checks.
• Establishing tests here sets the pattern for future modules and proves the value of the context abstraction.

Scope

• Add Vitest to the server package with a minimal config.
• Write unit tests around server/core/config.mjs to verify env variable overrides and normalization of booleans/SSL flags.
• Ensure tests run independently of the running server and do not mutate globals.

Tasks

• Add devDependencies to server/package.json: vitest, @vitest/coverage-v8 (optional), c8 (optional)
• Create server/vitest.config.mjs with test.environment='node'
• Add npm script: "test": "vitest"
• Implement a small helper to coerce env flags (e.g., toBoolean/toArray) and test them
• Write tests covering: - HTTP/HTTPS toggle normalization (server/web.mjs references) - DB SSL options type coercion (server/core/db.mjs references) - Env override precedence (config.yml vs process.env)
• Document how to run tests in server/README.md

Acceptance Criteria

• pnpm --filter server test passes locally
• Tests do not require spinning up the app or setting global.WIKI
• Clear examples of env normalization are captured in tests

References

• Vitest Getting Started: https://vitest.dev/guide/
• In-source tests (optional): https://vitest.dev/guide/in-source.html