feat(community): enrich feed & post-detail responses

[Rashed99Azm]

Summary

• Enrich feed response — CommunityFeedItemDto now includes DownvoteCount, TopicNameAr/En, IsExpert, IsWatchlisted, and VoteStatus (-1/0/1); all user-specific flags are populated from the current user's JWT and fall back to neutral defaults for anonymous requests
• Enrich post-detail response — GET /api/community/posts/{id} returns a new PostDetailDto with a nested PostAuthorDto (id, name, avatar, expert flag, posts count, follower count) plus topic names and user-specific flags; handler rewritten as a single JOIN query (one SQL round trip) and aligned with Response<T> + MessageFactory
• User activity counters — add PostsCount and CommentsCount to the User aggregate (same denormalized pattern as Post); maintained atomically in PublishPost, SoftDeletePost, CreateReply, and SoftDeleteReply handlers; migration AddUserActivityCounters adds both columns as int NOT NULL DEFAULT 0
• Feed sort & filter — new MostCommented sort option (PostFeedSort = 3); new optional PostType filter (Info/Question/Poll); PostType filter bypasses the Redis fast-path to SQL
• MessageFactory alignment — migrate GetMyFollows, ListPublicPostReplies, and ListPublicPostsInTopic handlers from raw return types to Response<T>; fix four endpoints that were calling Results.Ok/NotFound manually to use ToHttpResult()

Test plan

• GET /api/community/feed — anonymous request returns feed with topicNameAr/En, downvoteCount, isExpert, isWatchlisted=false, voteStatus=0
• GET /api/community/feed with JWT — isWatchlisted and voteStatus reflect the authenticated user's state
• GET /api/community/feed?sort=3 — results ordered by commentsCount descending
• GET /api/community/feed?postType=1 — only Question posts returned
• GET /api/community/posts/{id} — response shape has nested author object, topicNameAr/En, isWatchlisted, voteStatus; 404 returns standard error envelope
• Publish a post → author's postsCount increments; soft-delete it → decrements
• Create a reply → author's commentsCount increments; soft-delete the reply → decrements
• GET /api/community/topics/{slug}, /topics/{id}/posts, /posts/{id}/replies, /api/me/follows — all return standard Response<T> envelope (not raw JSON)