feat: add guest comment support

[OXeu]

Summary

Allow unauthenticated users to leave comments on articles. Guest commenters provide a name (required) and optionally an email and website URL.

Changes

Database

• Migration 0010.sql: Rebuild comments table to make user_id nullable; add guest_name, guest_email, guest_website, approved columns
• Test fixture: Update in-memory DDL to match new schema

Schema & Types (packages/api)

• Comment.user is now nullable; add guestName/guestEmail/guestWebsite fields
• CreateCommentRequest accepts optional guestName/guestEmail/guestWebsite

Server (server/src/services/comments.ts)

• POST /:feed: Authenticated users post as before; unauthenticated requests require guestName, accept optional guestEmail/guestWebsite
• GET /:feed: Guest comments return user: null + guest fields; logged-in comments unchanged
• DELETE /:id: Admins can delete any comment; users can only delete their own

Client (client/src/page/feed.tsx)

• CommentInput: When not logged in, shows guest form (name/email/website) + textarea, controlled by client config comment.guest.enabled (default: on)
• CommentItem: Displays guest name with optional website link; handles user: null gracefully
• Bugfix: Guest comment commenterAvatar now falls back to /avatar.png instead of empty string, preventing TypeError: e.user is null on some render paths

i18n

• Add anonymous + 4 comment.guest_* keys to zh-CN, en, ja, zh-TW

Tests

• 4 new test cases covering guest comment creation, validation, and listing
• Existing auth test updated (unauthenticated without guestName → 400, not 401)

[github-actions]

🚀 Preview deployed!

🔗 https://rin-server-pr-515.xeu.workers.dev

[github-actions]

🧹 Preview deployment has been cleaned up.

The temporary preview environment for this PR has been deleted.

[OXeu]

solves #514