Add nested posts UI (#5)

## Summary
- Cut hunt, question, submission, voting, hiding, and profile flows over to unified posts
- Render nested replies with permission-aware inline Reply/Edit controls and edited markers
- Add post profile queries and the author index migration they depend on

Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/store/migrate.go

@@ -64,6 +64,9 @@ CREATE TABLE IF NOT EXISTS posts (
 		{name: "index post replies", sql: `
 CREATE INDEX IF NOT EXISTS idx_posts_parent_created
   ON posts(parent_id, created_at, id)`},
+		{name: "index post authors", sql: `
+CREATE INDEX IF NOT EXISTS idx_posts_author_created
+  ON posts(author_id, created_at DESC, id DESC)`},
 		{name: "index root posts", sql: `
 CREATE INDEX IF NOT EXISTS idx_posts_root_date
   ON posts(post_date, post_state)
@@ -117,6 +120,15 @@ CREATE INDEX IF NOT EXISTS idx_post_votes_post_id
 	return nil
 }
 
+func migratePostAuthorIndex(ctx context.Context, exec execContext) error {
+	if _, err := exec.ExecContext(ctx, `
+CREATE INDEX IF NOT EXISTS idx_posts_author_created
+  ON posts(author_id, created_at DESC, id DESC)`); err != nil {
+		return fmt.Errorf("idx_posts_author_created: %w", err)
+	}
+	return nil
+}
+
 func migratePostDate(ctx context.Context, exec execContext) error {
 	steps := []struct {
 		name string
@@ -306,6 +318,7 @@ CREATE TABLE IF NOT EXISTS schema_migrations (
 		{"005_post_vote_post_id_index", migratePostVoteIndex},
 		{"006_post_date", migratePostDate},
 		{"007_post_state", migratePostState},
+		{"008_post_author_index", migratePostAuthorIndex},
 	}
 	for _, m := range migrations {
 		if applied[m.version] {