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_posts_test.go

@@ -97,6 +97,12 @@ VALUES ('homeowner', 'question-1', 1);`); err != nil {
 	if err := migratePosts(ctx, conn); err != nil {
 		t.Fatalf("migration is not idempotent: %v", err)
 	}
+	if err := migratePostVoteIndex(ctx, conn); err != nil {
+		t.Fatal(err)
+	}
+	if err := migratePostVoteIndex(ctx, conn); err != nil {
+		t.Fatalf("post vote index migration is not idempotent: %v", err)
+	}
 
 	var postCount, voteCount, legacyQuestionCount, legacyAnswerCount int
 	if err := conn.QueryRowContext(ctx, "SELECT count(*) FROM posts").Scan(&postCount); err != nil {
@@ -120,6 +126,23 @@ VALUES ('homeowner', 'question-1', 1);`); err != nil {
 			legacyAnswerCount,
 		)
 	}
+	var postVoteIndexCount int
+	if err := conn.QueryRowContext(ctx, `
+SELECT count(*)
+FROM pg_indexes
+WHERE schemaname = current_schema()
+  AND tablename = 'post_votes'
+  AND indexname = 'idx_post_votes_post_id'`).Scan(&postVoteIndexCount); err != nil {
+		t.Fatal(err)
+	}
+	if postVoteIndexCount != 1 {
+		t.Fatalf("post vote index count = %d, want 1", postVoteIndexCount)
+	}
+	if _, err := conn.ExecContext(ctx, `
+INSERT INTO post_votes (user_id, post_id, value)
+VALUES ('homeowner', 'question-1', -1)`); err == nil {
+		t.Fatal("duplicate user/post vote unexpectedly succeeded")
+	}
 
 	var rootParent sql.NullString
 	var rootAuthor, title, rootBody, city, postDate, rootCreated, rootUpdated string
@@ -210,6 +233,13 @@ INSERT INTO posts (
 		thread[2].ID != "follow-up" {
 		t.Fatalf("recursive thread = %+v", thread)
 	}
+	nonRootThread, err := queries.ListPostThread(ctx, "answer:question-1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(nonRootThread) != 0 {
+		t.Fatalf("non-root thread lookup returned %+v", nonRootThread)
+	}
 	if n, err := queries.UpdatePost(ctx, sqlc.UpdatePostParams{
 		ID:        "follow-up",
 		Body:      "The drip continues.",
@@ -238,6 +268,60 @@ INSERT INTO posts (
 		roots[0].UserVote != 1 {
 		t.Fatalf("root annotations = %+v", roots)
 	}
+
+	if _, err := conn.ExecContext(ctx, `
+DROP INDEX idx_posts_root_date;
+ALTER TABLE posts RENAME COLUMN post_date TO hunt_date;
+CREATE INDEX idx_posts_root_hunt
+  ON posts(hunt_date, hidden)
+  WHERE parent_id IS NULL;`); err != nil {
+		t.Fatal(err)
+	}
+	if err := migratePostDate(ctx, conn); err != nil {
+		t.Fatal(err)
+	}
+	if err := migratePostDate(ctx, conn); err != nil {
+		t.Fatalf("post date migration is not idempotent: %v", err)
+	}
+
+	var postDateColumnCount, huntDateColumnCount, rootDateIndexCount, legacyIndexCount int
+	if err := conn.QueryRowContext(ctx, `
+SELECT
+  count(*) FILTER (WHERE column_name = 'post_date'),
+  count(*) FILTER (WHERE column_name = 'hunt_date')
+FROM information_schema.columns
+WHERE table_schema = current_schema()
+  AND table_name = 'posts'`).Scan(&postDateColumnCount, &huntDateColumnCount); err != nil {
+		t.Fatal(err)
+	}
+	if err := conn.QueryRowContext(ctx, `
+SELECT
+  count(*) FILTER (WHERE indexname = 'idx_posts_root_date'),
+  count(*) FILTER (WHERE indexname = 'idx_posts_root_hunt')
+FROM pg_indexes
+WHERE schemaname = current_schema()
+  AND tablename = 'posts'`).Scan(&rootDateIndexCount, &legacyIndexCount); err != nil {
+		t.Fatal(err)
+	}
+	var migratedPostDate string
+	if err := conn.QueryRowContext(ctx, `
+SELECT post_date FROM posts WHERE id = 'question-1'`).Scan(&migratedPostDate); err != nil {
+		t.Fatal(err)
+	}
+	if postDateColumnCount != 1 ||
+		huntDateColumnCount != 0 ||
+		rootDateIndexCount != 1 ||
+		legacyIndexCount != 0 ||
+		migratedPostDate != "2026-08-26" {
+		t.Fatalf(
+			"post date migration columns=%d legacy_columns=%d indexes=%d legacy_indexes=%d date=%q",
+			postDateColumnCount,
+			huntDateColumnCount,
+			rootDateIndexCount,
+			legacyIndexCount,
+			migratedPostDate,
+		)
+	}
 }
 
 func TestMigratePostsReportsStep(t *testing.T) {