refactor: use Effect config for HttpApi authorization

[kitlangton]

Summary

• Add a reusable ConfigService.Service helper that turns Effect Config definitions into a typed Context service with generated defaultLayer and test layer(...) helpers.
• Define ServerAuthConfig with the new helper so env var names/defaults live in one Effect Config definition.
• Update authorization tests to provide typed auth config directly instead of mutating Flag or constructing env-shaped config providers.

Tests

• bunx prettier --write packages/opencode/src/effect/config-service.ts packages/opencode/src/server/routes/instance/httpapi/middleware/authorization.ts packages/opencode/src/server/routes/instance/httpapi/server.ts packages/opencode/test/server/httpapi-authorization.test.ts
• bunx oxlint packages/opencode/src/effect/config-service.ts packages/opencode/src/server/routes/instance/httpapi/middleware/authorization.ts packages/opencode/src/server/routes/instance/httpapi/server.ts packages/opencode/test/server/httpapi-authorization.test.ts
• bun typecheck from packages/opencode
• bun run test -- test/server/httpapi-authorization.test.ts
• bun run test -- test/server/httpapi-authorization.test.ts test/server/httpapi-instance-context.test.ts test/server/httpapi-workspace-routing.test.ts test/server/workspace-proxy.test.ts
• push hook: bun turbo typecheck