feat: @mentions in comments and posts with notifications and username autocomplete

[kingRayhan]

Summary

Support @username mentions in comments and posts (articles / diary bodies). When a user is mentioned, they should receive a notification. While typing @, show a dropdown that suggests matching usernames (search-as-you-type).

Motivation

• Surface relevant people in discussions and collaborative editing.
• Align with familiar patterns from GitHub, Slack, etc.

Scope (suggested)

Where mentions work

• Comments (CommentSection / comment create flows for articles, gists, etc.).
• Article body (editor + rendered markdown if we parse mentions server-side for notifications).

UX

• Trigger autocomplete when user types @ followed by optional query (debounced).
• Dropdown: avatar, display name, @username; keyboard navigation; select inserts @handle or normalized token agreed by product.
• Avoid firing inside code fences / Markdoc {% %} if feasible (parity with “no linkify in code” behavior).

Backend / data

• User search API or server action: prefix (or fuzzy) match on username / name, rate-limited, auth-required where appropriate.
• Persistence: store mention targets on comment (and article revision if mentions in body) — e.g. JSON array of user_id or join table comment_mentions.
• Notifications: extend existing notification pipeline (notifications.actions, types in domain-models) with e.g. MENTION_IN_COMMENT / MENTION_IN_ARTICLE (exact naming TBD), dedupe idempotency similar to other notification types.

Out of scope (optional follow-ups)

• Email digest for mentions.
• Mentioning teams or @here.

Acceptance criteria (draft)

• Typing @ in a supported composer shows a suggestion list populated from the backend.
• Selecting a user inserts a stable mention syntax that survives save/load.
• On submit, mentioned users receive an in-app notification (and unread count updates if applicable).
• Mentioned users who are not allowed to view the resource do not receive notifications (or receive no link — product decision).

References

• Notifications: src/backend/services/notifications.actions.ts, MyNotificationsFeed, NotificationType in domain models.
• Comments: comment section components and related server actions.