feat(life): add comments and replies to life posts

Introduce life_post_comments table for nested comment threads
Add API endpoints to create, reply to, and delete comments
Implement frontend UI with engagement counts and collapsible threads

backend/db/schema.sql

@@ -134,6 +134,24 @@ ALTER TABLE life_posts DROP COLUMN IF EXISTS link_title;
 CREATE INDEX IF NOT EXISTS life_posts_created_at_idx
   ON life_posts(created_at DESC, id DESC);
 
+CREATE TABLE IF NOT EXISTS life_post_comments (
+  id integer GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
+  post_id integer NOT NULL REFERENCES life_posts(id) ON DELETE CASCADE,
+  parent_comment_id integer REFERENCES life_post_comments(id) ON DELETE SET NULL,
+  body text NOT NULL CHECK (length(body) BETWEEN 1 AND 1000),
+  created_by_user_id integer REFERENCES users(id) ON DELETE SET NULL,
+  deleted_by_user_id integer REFERENCES users(id) ON DELETE SET NULL,
+  deleted_at timestamptz,
+  created_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS life_post_comments_post_idx
+  ON life_post_comments(post_id, created_at, id);
+
+CREATE INDEX IF NOT EXISTS life_post_comments_parent_idx
+  ON life_post_comments(parent_comment_id, created_at, id);
+
 CREATE TABLE IF NOT EXISTS skills (
   id integer GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
   name text NOT NULL UNIQUE,