Simplify post listing queries.

Use named sqlc arguments, scope vote aggregation to selected roots, join viewer votes directly, and add the indexes and drift migration required by the resulting access paths.

internal/store/migrate.go

@@ -104,6 +104,55 @@ ON CONFLICT (user_id, post_id) DO NOTHING`},
 	return nil
 }
 
+func migratePostVoteIndex(ctx context.Context, exec execContext) error {
+	if _, err := exec.ExecContext(ctx, `
+CREATE INDEX IF NOT EXISTS idx_post_votes_post_id
+  ON post_votes(post_id)`); err != nil {
+		return fmt.Errorf("idx_post_votes_post_id: %w", err)
+	}
+	return nil
+}
+
+func migratePostDate(ctx context.Context, exec execContext) error {
+	steps := []struct {
+		name string
+		sql  string
+	}{
+		{"rename post date", `
+DO $migration$
+BEGIN
+  IF EXISTS (
+    SELECT 1
+    FROM information_schema.columns
+    WHERE table_schema = current_schema()
+      AND table_name = 'posts'
+      AND column_name = 'hunt_date'
+  ) AND NOT EXISTS (
+    SELECT 1
+    FROM information_schema.columns
+    WHERE table_schema = current_schema()
+      AND table_name = 'posts'
+      AND column_name = 'post_date'
+  ) THEN
+    ALTER TABLE posts RENAME COLUMN hunt_date TO post_date;
+  END IF;
+END
+$migration$`},
+		{"drop legacy root date index", `
+DROP INDEX IF EXISTS idx_posts_root_hunt`},
+		{"create root date index", `
+CREATE INDEX IF NOT EXISTS idx_posts_root_date
+  ON posts(post_date, hidden)
+  WHERE parent_id IS NULL`},
+	}
+	for _, step := range steps {
+		if _, err := exec.ExecContext(ctx, step.sql); err != nil {
+			return fmt.Errorf("%s: %w", step.name, err)
+		}
+	}
+	return nil
+}
+
 type execContext interface {
 	ExecContext(ctx context.Context, query string, args ...any) (sql.Result, error)
 	QueryContext(ctx context.Context, query string, args ...any) (*sql.Rows, error)
@@ -151,6 +200,8 @@ CREATE TABLE IF NOT EXISTS schema_migrations (
 		{"002_user_profile_columns", migrateUserProfileColumns},
 		{"003_user_email", migrateUserEmail},
 		{"004_posts", migratePosts},
+		{"005_post_vote_post_id_index", migratePostVoteIndex},
+		{"006_post_date", migratePostDate},
 	}
 	for _, m := range migrations {
 		if applied[m.version] {