fix: GET /api/auth/me never returns 401 to avoid Basic Auth credential reset

[perber]

Unauthenticated requests to /api/auth/me previously returned 401, causing browsers behind a Basic Auth reverse proxy (e.g. Traefik basicAuth) to discard their cached credentials. Moves the endpoint to an OptionalAuth group that returns 200+null when no session is present. Adds Cache-Control: no-store to prevent reverse proxies from caching the identity response. Preserves the frontend session on 5xx/network errors (only clears on 4xx).

Closes #1013

[Copilot]

Pull request overview

This PR adjusts auth bootstrapping so /api/auth/me can be safely called without a session, avoiding browser Basic Auth credential resets while preserving identity freshness and cache safety.

Changes:

• Adds optional auth handling for /api/auth/me, returning 200 with null for unauthenticated callers.
• Adds no-store cache headers and backend/e2e coverage for authenticated and unauthenticated /auth/me.
• Updates frontend auth bootstrap error handling to preserve sessions on server/network failures.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
ui/leafwiki-ui/src/lib/bootstrapAuth.ts	Uses typed API errors to clear sessions only on explicit 4xx auth failures.
ui/leafwiki-ui/src/lib/api/auth.ts	Allows fetchMe() to return null and propagates status via ApiError.
internal/wiki/wiki.go	Defers search indexing status finalization.
internal/wiki/auth/routes.go	Moves /auth/me to optional auth and adds no-cache identity response headers.
internal/http/router_test.go	Adds router tests for /auth/me null/user responses and cache headers.
internal/http/middleware/auth/current_user.go	Adds nullable user lookup helper.
internal/http/middleware/auth/auth.go	Adds optional authentication middleware.
e2e/tests/auth.spec.ts	Adds Playwright coverage for /auth/me status, body, and cache headers.
internal/http/middleware/auth/auth_test.go	Adds middleware tests for optional auth behavior.

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