fix: unlike noop stale struct + likes floor at 0

- unlike noop now reloads tweet from DB (same fix as like noop from prev loop)
- decrement_likes uses GREATEST(likes - 1, 0) to prevent negative counts
- add fix_plan.md to track remaining work

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fix_plan.md

@@ -0,0 +1,25 @@
+# Fix Plan
+
+## Completed
+
+- [x] `tweet_like` tests: `user_fixture` missing `authorize?: false`, `Ash.Query.filter` needed `require Ash.Query`, `Ash.ForbiddenField.forbidden?/1` doesn't exist (use `match?`), `like` noop returned stale tweet struct → fixed all
+
+## In Progress / Next
+
+- [x] `unlike` noop returns stale tweet struct — same issue as `like` noop; reload from DB
+- [x] `decrement_likes` can go below 0 — use `GREATEST(likes - 1, 0)` via SQL fragment
+
+## Backlog
+
+- [ ] No pagination on user list (`/users`)
+- [ ] No CHECK constraint on `likes >= 0` at DB level (low priority, app logic prevents it)
+- [ ] `read :following_feed` returns error if actor is nil — should be policy-guarded
+- [ ] No search for users or tweets
+- [ ] Missing test coverage: follow/unfollow, comments, tweet creation, auth flows
+
+## Notes
+
+- Stack: Elixir/Phoenix + Ash Framework + React/TypeScript
+- Tests: `mix test` — 10 tests, all should pass
+- Build: `mix precommit` alias runs compile + test + format checks
+- No ClickHouse in test env (expected, non-fatal errors in test output)