Implement post likes functionality and update documentation

[edigar]

Description

Implements the README TODO: "Register who liked a post, so that each user can only like each post once, in addition to having information on who liked each post."

Likes were a denormalized counter on posts (likes int), incremented with UPDATE posts SET likes = likes + 1 — with no idea who liked and no deduplication (a user could like the same post repeatedly and inflate the count; the like/unlike handlers didn't even read the authenticated user). This PR replaces that counter with a normalized post_likes join table, so each user can like a post at most once, the count is always accurate, and we can list who liked a post.

Checklist

• Tests has been changed.
• All tests pass.
• Documentation has been updated.
• Code follows style guidelines.
• All merge conflicts have been resolved.
• The pull request is targeted to the correct branch.

Related Issue

Closes the "Register who liked a post…" item in the README TODO. No tracking issue.

Proposed Changes

• Schema: new post_likes(post_id, user_id, created_at) join table mirroring followers — composite PRIMARY KEY (post_id, user_id) enforces one-like-per-user at the DB, FKs ON DELETE CASCADE. The posts.likes column is dropped. New goose migration + build/initdb.sql updated.
• Entity: Post gains LikedByMe bool (json:"likedByMe"); Likes is now a derived value.
• Repository: Like (INSERT ... ON CONFLICT DO NOTHING), Unlike, Exists, FetchLikers; read queries derive likes (COUNT subquery) and liked_by_me (EXISTS subquery) instead of reading a stored counter.
• Usecase: LikePost/UnLikePost keyed by the authenticated user; GetLikers; new ErrPostNotFound; current user threaded into GetById/GetUserPosts.
• Controller/routes: LikePost/UnlikePost now extract the token user (this fixes the previously commented-out auth on like), map a missing post to 404, and return 204; new GET /api/post/{postId}/likes; GetPost/GetUserPosts return likedByMe.
• Docs: README TODO ticked off.

API contract

Method	Path	Auth	Success	Errors
POST	/api/post/{postId}/like	yes	204	400 invalid id · 401 no token · 404 post not found
POST	/api/post/{postId}/unlike	yes	204	400 invalid id · 401 no token
GET	/api/post/{postId}/likes	yes	200 []User	400 invalid id · 500

Post reads (GET /api/post, GET /api/post/{postId}, GET /api/user/{userId}/posts) now include the derived likes count and a per-user likedByMe flag. Like/unlike are idempotent (liking an already-liked post or unliking a not-liked post is a no-op → 204).

Tests Performed

Run on Go 1.25 (PowerShell):
go build ./... # ok
go vet ./... # ok
go test ./... # all packages PASS
Usecase and controller layers are covered with mocks (success paths, 401/403/404/400/204/200 mapping, ErrPostNotFound, likers listing, likedByMe). Consistent with the project's convention, the repository layer has no unit tests; its new SQL is verified by integration (bring up docker-compose, then like the same post twice → count stays 1; a second user → 2; GET .../likes lists both; unlike is idempotent; like a missing post → 404).

Additional Notes

• Behavior change: posts.likes is dropped and its (unreliable) values discarded; the count is derived from post_likes. Like/unlike now require and use the authenticated user.
• Follow-ups (out of scope): add the new GET /api/post/{postId}/likes row to the README Usage table; make usecase.GetById return the repository error instead of swallowing it; empty list reads serialize null rather than [] (pre-existing pattern).