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.

schema.sql

@@ -74,6 +74,9 @@ CREATE TABLE IF NOT EXISTS post_votes (
   PRIMARY KEY (user_id, post_id)
 );
 
+CREATE INDEX IF NOT EXISTS idx_post_votes_post_id
+  ON post_votes(post_id);
+
 CREATE TABLE IF NOT EXISTS sessions (
   token TEXT PRIMARY KEY,
   data BYTEA NOT NULL,